As schumigirl and damo said above, park to park tickets are a must to ride the Hogwarts Express between the two Potter parks.
Be at the park at rope drop. if you were stating onsite at one of the Universal resorts you would would get in one hour before park guests who are not staying onsite.
Consider costing out getting a room at Royal Pacific, Hard Rock or Portofino Bay for one night. This would allow everyone in your family of 5 to get unlimited express pass usage for your two days in the park. The passes are good the full day the day you check in and the full day the day you check out.
Unilimited Express is not usuable on Forbidden Journey in Hogwarts or Escape from Gringotts in Diagon Alley or Hogwarts Express. That being said if you want to enjoy the other rides while there and the parks are busy having the unlimited express pass will make riding the other rides easier.
Also think about one person getting an AP. This would help with discounts on tickets and merchandise and food. You can read about the benefits of all the AP's on the Universal Orlando site.
Now to a few tidbits on HP.
Attending the Ollivanders wand show is a must. You have two options. There is one in Hogsmeade and one in Diagon Alley. I like the one in Diagon Alley as it seems to be the least crowded of the two shows.
Ride the Hogwarts Express in both directions as the experience is different.
Make sure to allow plenty of cash for Potter treasures. Interactive wands are a must to do the magic tricks in both Potter parks and there are plenty of wands to choose from. There are TM's to help often with magic tricks.
The TM's are very kind and helpful. Everyone I have encountered in my many visits to Universal, from the resort employees to the TM's on the rides and in the shops to the restaurant staff are exceptional, kind, lovely people.
A lot of Potter fans like to wear the t-shirts of their fav houses of Hogwarts. You will also see the true diehard Potter fans young and old wearing their robes, school ties, scarves etc. All of these are available for purchase and more.
You will have to have meals at the Three Broomsticks in Hogsmeade and the Leaky Cauldron in Diagon Alley.
The family feast is a bargain for a family meal at Three Broomsticks.
Butterbeer at the Hog's Head (right next door to Three Broomsticks).
If you want to stay in the Potter story you will want to get your wands at Ollivanders and your robes at Madam Malkin's robes. Shop your furry creatures at the Magical Menagerie. Your Weasley joke things at Weasleys Wizard Wheezes and a skull or two at Borgin and Burkes in Knockturn Alley. All thes shops ares in Diagon Alley.
Get some great pics of the fire breathing Dragon on top Gringotts.
Make sure to go all the way through Knockturn Alley.
Have a look at the Parseltongue snake in the outside window of the Magical Menagerie. When your facing the store it's to your left. A lot of people miss this. It's cool and creepy.
This will get you started have a great time at Universal. You will absolutely love the HP parks. I never get tired of them. I always notice something new
