Lots of good ideas already. I made some shirts for my girls, the name fills with their favorite character. I also made the passport books and pressed penny containers. We made a Disney savings jar from an old oatmeal container, might be cute to do to save up some spending money as someone else said. Since I am planning our next trip so far in advance I am teaching my girls some of the basics of sewing for some cute projects for our next trip.
We bought our autograph books, I just couldn't figure out a good way to attach photos myself where the pictures would be protected like they are in the ones you buy (and the ones you buy have a loop for your marker as well).