You're going to love it, first of all. The whole place is amazing, the HP worlds even more so than anything else.
One thing - please don't rush through the rest of the parks. There is so much ELSE amazing that you really don't want to miss it. For me, those are slightly less about theming and more about the actual rides than the HP stuff, but they are really great. Our favorites (outside HP) were Men In Black (my high score: 474,000 - we're really competitive), The Mummy, and The Hulk. Give yourself time to really enjoy these.
I guess my advice would be to use early entry to get a head start on the things in the HP areas that don't allow express passes, hang out there until it gets really crowded (late morning?) then loop whichever park you're in (both are basically loop shaped making your decision basically just right or left), and back to HP around dinner time. Do make sure to see HP lands in daylight and at night. There's so much to see and it's all so well done. Also, don't forget to ride the Hogwarts Express. It's really cool.
Have a great time!
I won't rush through them, I'm just as excited to experience the rest of the parks! That's kind of why I was having the dilemna, I'm hoping the rest of our party will be just as excited to see and experience. I'm a reader and a planner, so I've listened to podcasts, read books and forums - I like to know what the best rides are and what to expect! I've heard that WW at night is beautiful!
I just did exactly this last October! Early access if for Diagon Alley only. Take advantage of it. First early morning day, head straight to Escape from Gringotts, afterward explore some of the shops in Diagon Alley, the are virtually empty during the morning. NOTE: Hogwarts Express does not operate until the general opening of the park and are not part of the early morning access. About 10 minutes before the park opens head to Hogwarts Express and get in line and take it over the Hogsmeade at Island of Adventure, head straight to Forbidden Journey ride at Hogwarts. After riding the ride head to the castle tour line ( I don't know when your breakfast reservations are at the Three Broomsticks, but hopefully you can fit in after the castle tour. The castle tour line will take you through the castle with no rush, you can experience each room in the castle at your leisure and take as many picture as you want! After that ride Flight of the Hippogriff, it is still fun as an adult and then if your more adventurous the do the Dragon Challenge roller coaster. It should now be about 11:30 AM, take your time to circle Island of Adventure the rest of the day. Starting with Jurassic Park and going counter clockwise. Once you meet back up to the Hogwarts Express at the end of the day, take it back to Universal, meander around London and head to Diagon Alley to end your day.
Day Two Breakfast Leakey Cauldron.....Head to Diagon Alley for the early morning access again and visit Ollivanders Wands, then you can explore the shops again, or ride Escape from Gringotts again. Eat Breakfast and then you can tour the rest of London area and Universal parks once Diagon Alley gets full. If you have the express pass getting to do everything at Universal is absolutely doable in one day.
NOTES: Diagon Alley, make sure you don't miss Knockturn Alley. Try all of the specialty drinks, we each got a different one and shared, Skip the Gillywater, it is just bottled water REALLY Expensive bottled water. DO get ice cream I actually like the caramel apple flavor better than butterbeer.
Drinks common to both Diagon Alley and Hogsmeade: Pumpkin Juice and Butterbeer(you can get these with breakfast as well). Try the butterbeer FROZEN, it is the best. Make sure all the girls go into the bathroom at Hogsmeade to hear Moaning Mertle! NOTE: if the restroom is really busy and noisy she is hard to hear.
Note on water rides at Islands of Adventure: You get really wet, but they are really fun. My brother and I have a system, one day we change into swimsuits with shorts and t shirts and ride all of the water rides then head to the hotel and hit the pool. The order from least wet to most wet is: Jurassic Park, Dudley Do Right Rip Saw Falls and last but certainly wettest Popeye's and Bluto Barges. If you don't have the time to change clothes or aren't staying onsite, Univesal has people sized air dryers. They fit a couple of people and cost $5 and blow warm air on you. Since I have not used them I don't know how well they work.
Thanks for the idea of a plan, that helps a bit! As of now, it looks like only IOA / Hogsmeade has early access but I'm hoping to get a USF day as well during our trip. I'm a coaster lover, so I don't plan to skip any of them, espeically Dragon Challenge! We won't miss a single thing in the WW, my cousin plans to explore every nook and cranny! I've actually had the regular and frozen butterbeer and preferred the regular

We visited IOA and Hogsmeade during a WDW trip about 2 1/2 years ago but we had the kids so I didn't get to spend a lot of time there.
I'm not huge fan of wet rides but I've heard that some of the Universal ones are great, so I'm going to suck it up and try them!
First thing, especially with the Hogwarts Express, you can almost think of USF & IOA as 1 big park. Park Hopping is easy. And Big HP fans may want to ride the Hogwarts Express each direction more than once.
I would break things up and do 1/2 days in each park each day. With Express Passes, there isn't a lot to plan, just the rides that don't accept Express Passes:
Forbidden Journey
Escape from Gringotts
Hogwarts Express
Kudo and Kangs (flat ride in Simpsons that normally doesn't have a long ride)
Pteranadon Flyers. (Being an all Adult trip you can't ride this anyways. Adults need to ride with a child under a certain height).
The only other ride that can sometimes be a problem is Despicable Me. The biggest question is what park(s) will have early entry during your trip. I would try and hit the signature HP ride during Early Entry before heading over to breakfast. You can save Breakfast for regular park hours. My only other suggestion is to relax and slow down a little. Since you won't be waiting long for rides you'll have time to ride all the rides and do things like watch the Blues Brothers Shows or Celestina Warback and the Banshees.
Have fun.
Thanks! That's what I'd like to do, is just try to take it slow and enjoy it - it never works out that way, though! I'm used to WDW and usually do the parks commando style!

Its usually only my kids and I, though, so having 3 other adults with me may slow me down a bit! I can't exactly throw them in the strollers and rush to the next ride! I was really looking forward to the Tri Wizard show but it looks like its not showing during our trip.
The beauty of Us, unlike WDW, is that you REALLY don't need tot take the pre-planning approach at all. Nor do you need to stress out about missing any FP time windows.
First and foremost, RELAX!!!! Allow everyone to simply relax and explore.
the only preplanning we did - and it was kind of a mistake - was booking a preplanned lunch a week before we went at Mythos. Mythos is excellent, and it is good to reserve ahead- our mistake was having to rush to get over there. Our second visit, we just relaxed and meandered where and when we felt, much better. We just ate when we wanted.
EVERYONE make s a beeline for Diagon Alley as soon as they arrive. Enough said. Sure, as with any park, there's a benefit to arriving early, but zig when the crowd zags. If you want to see HP in the AM, do it...just know that's probably the worst the Gringott's line will be all day.
Since you are going in summer, Both areas are great after dark. If perchance youa re arriving to your hotel in the afternoon - it makes sense to add an extra partial day to your touring- and START with a PM visit to HP.
The wands are pricey, but Oh, so much fun!
Also, Universal doesn't allow bags on many rides. On our last visit, they went even further, and didn't allow ANYTHING in pockets either. The lockers are free while you ride, (except the flume) but it pays to carry the bare minimum. You WILL get rather wet on water rides, so be PREPPED.
Also, unlike WDW, US has the new advanced Coke product machines in the parks, so A.) you can get any flavor soft drink Coke makes, and water/ice - no wait. (if you have a cup/bottle)
Truly, we've been multiple times in the past year- and busy weeks no less. With FotLA, we mostly just went wherever we wanted whenever we wanted with only short waits for almost everything. As another poster said, the only other ride you might want to have a strategy for is Despicable Me. I suggest doing it first on one of your mornings. It is backed up even late, because it is at the front of the park.
Oh, after the parks close, the eateries in City walk see some traffic. Also, Breadbox and the new frozen Yogurt place were not open all that long after the parks closed. We partly picked both cause they had short lines compared to say, Moe's and Panda Express. I think the TS places were not as bad. The turkey sandwich was very good.
For breakfast, we did grab and go. For lunch, we ate at both HP places. Both were very good. Leaky C has a limited menu. HP ice cream place super. We had a little wait at 3 Brooms, but it wasn't too long. (that day we were bad and ate just at 12:15pm-I know better!)
If prime seating matters, then maybe arrive a little early for the late night show - but NOT like WDW parade.
DO see the Animal Show! Do take time to notice details all over US. The park is WAY nicer than it used to be. the landscaping is amazing, and many pre-rides have fun jokes.
always in summer, afternoon breaks from the heat are wise.
Its going to take a LOT of willpower not to plan like I do for Disney! I just booked an October trip for WDW and am trying to restrain myself from making touring plans this early! lol It was just also making me nervous because we are going in June, as a general rule, I don't do WDW in the summer, always in the off season, so I'm not sure what to expect as far as crowds. Our breakfast reservations are at 10am (the reservations came with out package) so hopefully this doesn't throw us off much, maybe I should make them earlier and it could be the last thing we do before heading to the rest of the park?
We will be checking in sometime in later afternoon, so I think park time will be out. Thinking to just do City Walk, maybe some pool time - let my mom rest before she has to run around after me in the parks! SHe came with me to Disney once and still tells stories about having to chase behind me while I run to the rides at rope drop (it was before
MDE and FP+)
I think we will do the wands, so having some nice relaxed time to do that would be fun - possibly at night? When would be the best time to explore that part?
Mom doesn't do coasters, so hopefully we can leave bags with her while we ride.
For the Coke machines, do you have to buy the US cups or use your own bottles? How does that work? How much would the cups be? not sure I'll want to carry that around all day...
thanks so much for all the tips!