I really don't know; with my luck I'd put something in and then realize a few days later that I needed it. Everything that I could possibly think of putting in is something I'd want/nned in the next fifty years.
I know! A cell-phone. In fifty years we can marvel over how giant cell-phones were in 2009.
Boy oy BOY, is that a long list! Let's see, nearly 30 and I still play video games; I have a general love of animation, from Disney to anime, especially shojo anime/manga; I'm also a big Harry Potter fan. And Hello Kitty. And My Little Pony.