I would watch the the prices at the Resorts that have Unlimited Express Pass included in your stay. Those would be 1. Portofino Bay, 2. Royal Pacific, 3. HardRock Hotel.
The Unlimited Express will give you front of line access to all the rides. If you go during a busy time this will help a lot.
Make sure you have Park to Park tickets. You have to have these to ride the Hogwarts Express between the 2 Potter Parks.
Remember as a onsite Resort guest you have Priority Seating at the restaurants at Park and CityWalk. So if you don't have reservations this will move you up over those not staying onsite.
Go to the "Wizarding World Touring Plans Details" Sticky located about 6 above and read ruthies12 information on all the hidden things to look for at Diagon Alley and Hogsmeade.
Find out which house of Hogwarts your daughter likes Ravenclaw, Hufflepuff, Slytherin or Gryffindor. Buy her a T-shirt of her favorite group, a Chocolate Frog (has famous pic of witch or wizard inside), a box of Bertie Botts, and a Interactive want of her favorite Potter character (so she can do the spells at both Potter Parks).
Make sure to take her for a meal at Three Broomsticks in Hogsmeade and The Leaky Cauldron in Diagon Alley.
Another special place for lunch is Mytho's in Islands of Adventure.
Contact the Resort your staying at and let them know you will be celebrating her 21 birthday. Let them know you would like to have a cake etc.... and they should be able to help you plan something special.
Pick up a birthday button at Guest Services!
Like this poster suggested stop by guest services for her Birthday Button.
If you have any more questions come back here and someone will jump in and help you.
