Since lending someone (not me - I always return them AND in the same condition I received it) a book is often a brave act, I often like to include a bookmark when I return it, as a thank you. I think something including a coloring bookmark would be a nice touch.
They would also make a nice little project for little ones (the difficulty level not being too high) and when they are done ... laminate them to make them more permanent.
Just thought I'd share.
From Classroom Doodles
From By Dawn Nicole