You can buy Hogwart's Acceptance letters on Etsy (or make them yourself if you have a computer and printer). I gave one to my DD rolled up and tied to a stuffed owl that I got really cheap on
amazon.
If you are going to have express passes then lanyards are a really good idea, you can buy them online from universal, they have all different characters and each of the different Hogwarts houses. You can also get them on amazon cheaper, as well as cheaper robes on amazon.
Universals website has all different gift cards you can buy, and they have ones with harry potter characters on the front. I gave one to my DD wrapped up with a note that said "The wand chooses the wizard" so she would know it was for her wand. Yes Olivander's takes Universal gift cards!