It looks like both Islands of Adventure and Universal Studios are going to have early park entry for the whole month of July. Early entry for both parks is at 7 am. Like above poster stated. Be there early as they will let you in ahead of time.
Check out the maps for both parks on Universal web site so you will have the lay of the land and an idea of what you want to do in each park.
There are two Potter Parks. Hogsmeade in Islands of Adventure and Diagon Alley at Universal Studios.
Escape from Gringotts would be my first choice. Then do Ollivanders. Also if you want to repeat ride Gringotts and you don't want to go through the full scenic line . You can go through the single rider line. This is my favorite ride.
Usually later in the evening Gringotts is fairly easy to get on.
After you do Escape from Gringotts and Ollivanders don't forget to wander around Diagon Alley and Knockturn Alley and take in all the neat shoppes.
I like the Ollivanders in Diagon Alley better. I have done the one in Hogsmeade numerous times.
The Ollivanders in Diagon Alley has an area that you can wait in inside and it never seems to be as long a line as the one in Hogsmeade. The interior also seems to be a little more spacious.
Agree with havoc315 Despicable Me Minion ride is very popular. This would be one ride to get done before lines and wait times get to bad.
Hopefully since you are doing a Sunday it won't be to bad a lot of folks check out on Sunday, but a lot of locals hit the park on the weekends. Never know for sure.
Have a great time

Trust me you will want to come back again and spend more time at Universal.