WWoHP is small, but there is so much to soak in. Damo mentioned some of my favourite things. We didn't even do Duelling Dragons, and dd only did Forbidden Journey once. We had flexpasses in 2011 and two weeks, so we came and went several times, just to wander around. Will do the same this time. (The only pain is paying for parking every visit!) Honestly, we're not thrill riders, so while we did enjoy some of the rides elsewhere in the park, they were filler on the way to Hogsmeade. And we're totally prepared to pay the price of admission for WW alone.
I would suggest getting there early. (We are "off site" people too, a "deal" for one room at $239/nt isn't a deal when we can rent a house with a pool for half that. And the only must-dos for us are in the Wizarding World, where the express pass isn't much use. I mention that because a lot of people love the on-site benefits--but if you don't have them, you can still have an amazing time.)
Start with Hogsmeade, don't be distracted by the shinies along the way. ;-) Hard with an 8 yo, I know! Even though the great masses can head in at general opening, we arrived for rope drop a couple of times and still managed to get a lot done before it got too crowded. I would also recommend coming back around suppertime, a few hours before close, if it gets too crowded to enjoy through the busy part of the day.
Will the 8 yo be ok with FJ? My dd was turning 8 when we went, and she hadn't then read the books, so we only got her on it once. The Dementors kinda freaked her out. (Me too, I'll admit, but it's worth it!) She loved the castle though. Your littlest can stay with your mom if it's not for him.
Flight of the Hippogriff should be great for the 8 yo, and it's so fun to see Buckbeak and Hagrid's hut. We didn't do Duelling Dragons, but we walked through to see the Triwizard cup and all the theming.
We fully intend to line up for Ollivander's over and over in hopes for dd to get picked. Ds did, and it's still one of his favourite memories, ever. Bar none. Seriously. I have told dd I will do it up to 7 times with her if we have to. I hope your little Potterhead gets picked, he even will get a chance to use a spell or two if he does. There are lots of discussions about how to get picked, but I would just go with "happy to be there", and polite.
I think the thing that makes it so amazing for those of us who love the books is that you feel like you are actually walking into a beloved "reality". The details totally support that, and there are so many, many details to see without ever going on a ride. Have a budget for butterbeer and Honeyduke's--we told my kids we were going as soon as we made the booking so they can save up their allowances! Allow yourself time to hang out at the Three Broomsticks, and pop into the Hog's Head if you just want a cool place to sit and sip another butterbeer later. (Or something stronger for Mama!) A lot of it is shops, but we didn't feel required to buy something in every store. The Monster Book of Monsters, the Owlery, the things moving around in the shop windows, all fabulous sight-seeing. The shop at the back (Dervish and Bange's maybe?) had some really cool "wand collections" for sale...we had no intention of spending $300 on wands, but to see all the Triwizard Champion's wands together was really cool, as were many of the other displays. The staff we met were all very well versed in Harry's world and were happy to talk about their favourite wands, or which house they had been sorted into, and so on.
There is lots to fill your days elsewhere in the two parks. (Are you getting park-to-park?) I think someone has mentioned Jurassic Park, which is made for little boys. My ds was mildly interested in the superhero stuff, but not enough to get on the big scary coasters with their long lines. We were more ET-speed. (Which is kinda like "Peter Pan" or something like that--old school, sweet, not fast or rough.) My kids enjoyed the shows/rides associated with characters they knew, like Shrek and MiB. The Animal Actors show is great too. We are looking forward to trying the Despicable Me attraction.
Good advice to go to the Universal website and see what appeals. You know what kinds of things your boys like, so you should be able to pin point some new and fun stuff for them.
Have fun! And come back and tell us about it!
Either way, I ate at Three Broomsticks for lunch in Dec and thought the wait was fairly quick for what it's worth. If he's a big Harry Potter fan I think that time is time well spent.
He is also pumped about the other attractions, too. He's just not a big coaster fan. 
