Ok, having just been there a week ago for our 3rd visit here’s my advice:
Start from Universal Studios side. This will lead you to Diagon Alley through the London Town area. I like to start this way because it’s very Potter to start in Diagon Alley.
At the end of the street is Gringots and the Dragon. This is also where the Escape from Gringots ride is. It’s very cool. (again, another reason to get the express pass)
The Leaky Cauldron restaurant is there, The Weasley’s shop, Olivander’s Wand shop, the Ice Cream shop and the Butterbeer store… right past the Butterbeer shop and across from the restrooms is a dark alley you can take back to Nockturn Alley and some neat stuff.
After you explore Diagon Alley and come back out, turn to your left and there is Grimold Place and you’ll see # 12. Be sure and stand there a while and on the second floor window you’ll get a surprise.
You will pass Kings Cross station on way to Diagon Alley and that’s where you can take the train to Hogsmead which is in Islands of Adventure. You want to take the trip on the train both ways as the experience is different each way. VERY MUCH WORTH IT.
In Islands of Adventure is Hogsmead is the Three Broomsticks Restaurant (identical menu to Leaky Cauldron) and the infamous Hagrid’s Motorbike ride. I’ve got no advice on Hagrid’s other than be ready to stand in line. I think that if you were at the park the minute it opened and went straight to the ride it might not be too bad but I don’t know. We tried but just couldn’t justify the time standing in line.
In Hogsmead is Hogwarts Castle and it’s pretty amazing to see and be ready to take pictures of. Inside the castle is FORBIDDEN JOURNEY. It’s a wild ride that you should look up descriptions of if anyone in the party has motion or issues on rides. Having said that, in our three day visit we went on it 13 times. Child #1 loves it and with ExpressPass we pretty much just walked on.
There’s also the mild and short Flight of the Hippogriff rollercoaster. Even kids that don’t want to ride coasters will ride it since it’s short and mild.
There is a wand shop in Hogsmead but not different than Olivanders in Diagon Alley. Honeydukes candy shop is here. There’s some unique items but a lot of what they sell is candy you could get at the bulk candy store in your local mall.
The big thing about both parts of the Universal Wizarding World is the experience. The immersive visit is what it’s all about. Just walking through Diagon Alley waiting for the Dragon to do his thing is one of my favorite things and being there at night is amazing.
Yes, you need express pass, it’s worth it especially at this time of year. I would never consider going to the park without it.
Other cool rides in the parks: The Minions Ride, Transformers, the intense Curse of the Mummy, 3D Race through New York and if anyone loves wild coasters there some of those.
Food-wise… Universal’s in park food options are nothing to get excited about.
Please feel free to ask as many questions as you like.