At first, we just did a little of this, a little of that. My mom had given us a bunch of small projects for the kids to do (making bracelets, painting sun catchers, etc.), so we started on those while I searched for a good book of ideas.
A couple years ago, I picked up a book called Mudworks by MaryAnn F. Kohl. I liked it a lot, but there wasn't much besides sculpting, exploring, and playing with different compounds. Well, last week, a friend of mine told me she had another art book by MaryAnn Kohl, Scribble Art. Turns out, it's exactly what I was looking for.
Remember making crayon rubbings? What about spatter painting? Finger painting? Collages? Scribble Art is based upon the premiss of exploration. There are no specific outcomes to the projects. What she does instead is give you a list of materials, the process used, and variations that you can do with the materials/processes. The final outcome is open ended. The best part, for me, is that the projects can be easily adapted to a wide range of ages, from preschoolers through adult. You can make the project as complex or as simple as you want. I can enjoy the art experience right along with my kids. Another bonus is that most of the materials used are found lying around the house (crayons, chalk, tape, yarn, glue, tissue paper, string, newspaper, baking supplies, etc.).
It's a great book. Check it out at your local library (I know Longmont has it). You can also "look inside" on Amazon.com here.