Jean Van't Hul has many wonderful ideas on her blog: The Artful Parent
The blog is full of photographs showing her art projects. She does them with her own children, as well as with groups of children. Many of the projects you can accomplish with materials you already have laying around the house.
Monoprinting project on The Artful Parent.
When looking for a craft project, blogs often have a lot of inspirational ideas. Gather your materials and spend a nice afternoon crafting with your little one. It could be messy, but also a lot of fun!