The one souvenir I'm looking forward to getting is a wand from Ollivanders. Just like in the books and movie, the wand will pick you. Can't wait to see how they make this happen.
We went to the travelling HP exhibit last weekend. The attraction emptied into a gift shop at the end -- and I would think the souvenirs would be very similar. The wands started at $45 -- but perhaps they will have a less expensive option in the theme park. They had a few kinds of candy -- frogs, beans, etc., a lot of toys and action figures, plenty of collectibles, books, t-shirts, sweatshirts, postcards, posters, bookmarks -- there were all kinds of things, and I believe you will be pleased with the selection!