So far I've posted a few DIY ideas on bookmarks - Ribbon Bookmarks, Fabric Bookmarks and fun book corners parts 1 - 2 - 3 - 4. Here's another fun project you can do with a bunch of cheap materials - paper clip bookmarks.
Comes with a tutorial (link at the bottom of the post)!
You can use paper, fabric, buttons or polymer clay to name just a few ideas. You can pick out materials that are already colorful and have printed patterns or you can draw your own like seen on picture one.
Tutorial for a string (or ribbon) paper clip bookmarks