My 3 year old will sit for hours making collages. You can use any hard board - matte board, foam board or even wood - and a bottle of craft glue from Michaels ($1.29). You can do this after a nature walk where you gather natural objects for the collage or after a trip to the dollar store. Here are some photos: http://2002music.com/collage.html
It's even more fun when you throw an old vinyl tablecloth on the floor and invite friends to make collages for an "art contest". Prizes are for "tallest" (must be able to be turned upside down though), most creative, creepiest, etc.
I often like to do this one in the kitchen where I can float in and out of the area, cleaning up and cooking dinner...