Well, these aren't nearly as complete as tara's idea, but I was thinking: scavenger hunt, tea party, hide and seek in the shelves, build a fort out of books, go exploring and map the wilderness (or genre sections, as you will), archeology (find and study the oldest books), use book titles to write a story, find a book that has someone like them as a character, dress up/disguise themselves as each other, play board games, play twister (that'll make for some interesting pictures), discover they're locked in and search for an escape route, slumber party, um, um... yeah, that's all I got.