All sorts of stuff! My ADR list w/confirmation numbers. A list of park hours/fireworks/parades. All my resort reservations printed out. A trip tik for the long car ride, as well as some other maps. A sheet I made with all the countries in the World Showcase with a space that says "ate" and "drank", in case we eat or drink around the world - it makes for a good scrapbooking page. I have my little penny press collector thing in one of the plastic holders, although it's kind of bulky.
Matt actually started this trend! I think it was our 1st anniversary that he got a binder, and used all my scrapbooking stuff to make a cover page. He totally understands my obsessive planning nature, haha.