So, after you get your two-park ticket, what to do in HP world? I'm assuming you're going for one day and staying off site.
Find out which park has the early opening. If you are staying offsite, go to the OTHER park. Go early. You will usually be allowed in a bit early and held at a rope after security. You want to be at the front of that line. It's pretty much the same as rope drop at Disney.
When you finally get to Hogsmeade or Diagon Alley, go immediately to the "big" ride (Escape from Gringotts or Forbidden Journey). Even without the front of line access, the theming is great throughout the line (especially at Gringotts), so not too bad if there's a bit of wait. If you are in Diagon Alley, you're done with HP rides. If you're in Hogsmeade, you could go on Flight of the Hippogriff. Be aware that Hippogriff is just a basic kiddie coaster with some HP theming thrown in, so you may want to skip it, depending on the lines.
After that, you'll want to soak up the atmosphere of HP world. First thing is to buy an interactive wand. They run about $50, and you can get a replica of a movie character's wand or an Olivander's Original unclaimed wand. You can walk right into Olivander's and buy one or you can go through the wand ceremony first. I highly recommend doing the wand ceremony, but lines can get long. If you're in Hogsmeade there's only one room for the wand ceremony whereas Diagon Alley has two or three, so the lines are paced accordingly. There will probably be quite a line by the time you're out of the first ride, so my advice would be to go buy a wand from Olivander's so you can play at the windows right away, and then go back to experience the ceremony in the afternoon. I found that the lines for the ceremony were super long in the morning, but you could basically walk right in for the afternoon/evening. At the ceremony, one kid is picked to be matched with the wand (because "the wand chooses the wizard"), and there are some cool little special effects. The child can choose to buy the wand that "chose" him/her, but doesn't have to. In my case, ODS had his heart set on getting his wand that way, so went through six times over two days in both parks until it happened for him - hopefully no one in your party is that fixated. Take a look outside Diagon Alley where the theming is cool, too. The night bus has an interactive shrunken head and a conductor who are great to chat with.
The shops are fantastic. My kids bought their lanyards in Quality Quidditch supplies. There are "magic" elements in many of the shops, too. If you go to Madam Malkin's, there's a mirror that talks to you, and gives it's opinion on how you look. You can exchange your muggle money for Gringotts bank notes for use in the Wizarding World, and interact with an excellent animatronic goblin banker.
Look up the times for the shows, and plan to catch them. In Diagon Alley, there's an excellent stage version of The Tale of the Three Brothers, using puppets and adapted from Tales of Beadle the Bard. My kids and I agreed that this was the best of all the live performances. There's also Celestina Warbeck, a singer, with a motown/big band type vibe. I found it excellent, but my 7 and 9 year olds were less enthusiastic. In Hogsmeade, there's the Frog Choir, which is cute. The Triwizard Spirit Rally is worth seeing. The Beauxbatons ladies do a dance, and the Durmstrang guys do another dance that's part dance part martial arts. This also features a Hogwarts prefect who is hosting it, and has some nice schtick about muggle cameras.
Plan to ride the Hogwarts Express in both directions. It's a different experience each time, with different story lines. Enjoy the theming of the Kings Cross station, including the advertisements. There's a fantastic optical illusion that allows you to see people going through the wall to platform 9 3/4.
Since you're making a day of it, I found the line times to be similar to Disney in that they peak in the middle of the day and are lower in the early morning and late evening. You may want to return to Gringotts or Forbidden journey just before park closing for another ride when lines will be much shorter. One evening, ODS rode Forbidden Journey 2 or 3 times in row with almost no waiting, while YDS rode Flight of the Hippogriff over and over - just got off the exit and ran right back around to the front of the by-then-almost-non-existent line.
The other areas of the parks are worth a look, too. I mean, you're there, and you've bought your tickets and everything. If you go back to the main forums page, and scroll way down, you'll find the forums for Universal/SeaWorld, which might give you better trip planning advice than in the Disney Trip Planning Forums.
