Has to be something realistic (I'd love to go to Hogwarts too) but doesn't necessarily have to be something easily achievable (LET'S COLONIZE SPACE).
My biggest dream (since I was 13) was to join the Peace Corps. As I've gotten older, I'd really just love to go to India and do humanitarian work with orphans. Really any underdeveloped country with a big orphan population that needs love. I'd probably come home with 20 kids, but you know.
What is your biggest dream?