I'll throw in my 2 cents:
go to the bathroom in Hogsmead- next to 3 Broomsticks. "Public Conveniences". Moaning Myrtle is there
The seating outside 3 Broomsticks (in the back) is one of the best places to relax (if it's not too hot or raining).
The windows in the stores are incredibly detailed.
Don't be afraid of a 60 minute wait for FJ or Gringots- the queues are stunning and you'll wish you had more time.
Ride flight of the Hippogriff at night if possible- it's super silly.
I'm a beer girl and REALLY enjoy the stout in DA, but the Hog's Head brew in Hogsmead makes me feel like I'm home- it's yummy too.
Although you can buy wands pretty much anywhere, it is only acceptable to buy them in WWoHP.
Don't miss knockturn alley- best theming of the entire place, Jaws easter eggs are back there.
Florian Fortescue's ice cream is amazing- and worth the wait.
My favorite show is Beedle the Bard- The Tale of the Three Brothers. Stunning puppets and of course, a beautiful story.
We go all the time- we're lucky enough to live close-ish, and I STILL discover new things every time. Sorry you only have one day, the parks are far more then WWoHP, they are spectacular! Sounds like you're going before school lets out and after spring break, so fingers crossed it won't be busy
edit- like others have said- the locker situation by dragon challenge and FJ is horrible, be patient