One day just doing HP sections of the parks-
Start at Diagon Alley with Gringotts. Then I'd do Ollivanders and go to the Hogwarts Express. Once I was at Hogsmeade, I'd get in line for FJ. Afterwards, I'd explore the shops a bit, get some food at Three Broomsticks. Explore a bit more, catching the two shows if possible. Then I'd ride Dueling Dragons and Hippogriff (if you have kids). Then if the line didn't look too long, I might to FJ again (It's by favorite). Then get a butter beer and catch the train back to London. Explore Diagon Alley (lots to see there), catch the two shows, dine at Leaky Caldron for dinner and have ice cream at the ice cream parlor for dessert. Stick around for darkness (this area is great in the dark, esp Knockturn Alley and the dragon's fire. Then I'd do Gringotts again. Single rider if the line is long.