I used a lot of stuff that I had and never used, like pom-poms and various foam stickers. I also had a small container of beads and gems. There are also various crayons and markers that no longer have a home (or the other markers wore out) that I threw in the box. Some of the crayons are even from local restaraunts.
I bought a very big bag of foam stickers on clearance at Jo-Anns before Christmas. There are also plenty of craft supplies at your local dollar store (like pipe cleaners). Of course, what would a craft box be without paint and glue?!?!
I love this because she can just sit at the table and go crazy. Nothing was really expensive, and the whole point is to make a mess and have fun.