When I went I gave them super hero capes because "They're like my own personal superheroes" and Joe was super nice about it, but it really depends what you're into. If it's not fragile they'll be able to keep it longer.
If I were to do it again, I would do something personal, or something they can wear, but that's just me. If you're into, like, writing songs for the ukulele, take your instrument and play them a song. It's really just what you think they'll like and that you want to put effort into :)