I can easily spend an entire day just on the HP stuff and I've been several times. I swear I find new details every time I go. Here is a touring plan I've made, it's by no means complete and certainly does not have to be done in this particular order.
A Perfect Day in the Wizarding World
Start by being at the gates to the park at least a half hour before opening, they frequently allow regular day guests in early.
Start on the Universal Studios side.
Proceed directly to Diagon Alley and enter through the brick wall. Pause and listen while you go through the brick wall.
Go straight to Escape from Gringott’s and ride it (BEST RIDE EVER!).
When you exit the ride, go into the Gringott’s Money Exchange. Converse with the goblin working there while you exchange some muggle money for wizard money.
Go to Olivander’s, watch the wand ceremony, then purchase an interactive wand, use your wizard money to pay for it.
Go to the Leaky Cauldron and eat breakfast (get there before they stop serving breakfast at 1030)
- Try an Otter’s Fizzy Orange Juice, a Tongue Tying Lemon Squash, or pumpkin juice with breakfast
Walk around Diagon Alley casting spells, looking through the shops, and reading all the advertisements on the walls.
Special Effects list:
- Go in Madame Malkins and stand in front of the mirror
- Knock on the door of the Daily Prophet
- Stand on the corner between the restrooms and Gringott’s and listen for the wizarding wireless network
- Go in Borgin and Burkes and stand in front of the vanishing cabinet and listen
- Open your wand map while in Knockturn Alley under the black lights
Be sure to hit all the spell spots that are listed on the map, there are two that are not listed- look for the piece of parchment paper in the window of Scribbulus’s Shop and wave your wand at it and look for the potted plants in the apothecary’s shop and wave your wand at them.
Go into Shutterbutton’s and do the acting in front of the green screens, then purchase the DVD.
Stop at Eternelle’s Elixirs and pick up some potions to take home (you can skip the “gillywater”, it is just overpriced bottled water with a label, it is cheaper to take the potions home and mix them with regular water).
Stop by the Hopping Pot OR the Fountain of Faire Fortune and get a Fishy Green Ale (just get one and ask for more straws if you have more than one person, it is to everyone's taste).
Exit Diagon Alley to the London Waterfront.
In the London Waterfront:
-Stand in front of number twelve Grimmauld Place and look at the second story windows for several minutes.
-Go in the phone booth and dial the number to the Ministry visitor’s entrance (it’s written on the phone).
- Go into the back of the Knight bus and look inside. If a conductor is out front, talk and take pictures with him.
Enter the train station and ride the Hogwart’s Express to Hogesmeade.
Enter Hogwart’s and ride Forbidden Journey. If you have express passes, use the regular line at least once to see all the details.
Ride Flight of the Hippogriff.
Go in the Owl Post and mail a letter to yourself, it is a working post office with a special Hogsmeade postmark.
Go in Dervish and Banges and look for the monster book. Stand out front of the store and listen to all the different howler messages in the outside window.
Go in the bathroom and listen for a familiar voice.
Do all the spell spots in Hogsmeade.
Eat a late lunch/early dinner in the Three Broomsticks.
Go in the Hog’s Head and get Butterbeer (for real beer - the dragonscale is better than the wizard’s brew IMO), then go outside and up the street to the stage to listen to the frog choir or the Triwizard spirit rally.
Go in Honeydukes and get some sweets to take home (in the bakery case in the store the cauldron cakes are very good, they also have pumpkin pasties).
Ride the Hogwart’s Express back to Diagon Alley (the ride is different both ways).
Go to Florean Fortescue’s for an ice cream to finish off the night.
Be sure to stay in Diagon Alley until after closing, the rides and shops close but you can still walk around and look in all the lit-up windows and it is very beautiful at night.
