I would do the broomstick video as a birthday memento, it's really good
Make sure you book early, the tickets sell out fast
Go on to amazon (maybe eBay, not checked) and get cheap HP sweets before you go and take them with you, you can get 3x as much for your money even including postage, and can just surprise her with them at the end, save her wasting birthday money! I say this on every thread, but it really is a good tip in my opinion
Maybe ask family for £ or you give her £ as a gift if you think she would like a wand, they are £25
You can access gift shop at the start, you have to walk through it at the end. If she is going to get wand/Cape type things it may be a good idea to get them first so she use them on the day
Lastly, don't get the audio tour if it's your first trip, it may be good on later trips but there is already so much to see on your first trip. We got them and didn't use them, but having looked at it I would get it if when we go again to get more out of the second trip