I decided to purchase 3 art journals so that I could participate as well. This has helped my boys "buy-in" immensely. It's also increased the time we spend on the project so that no one (mainly Nate) is done in 2 minutes, leaving me frustrated at the terrible ratio of set up and clean up to time spent on a project. And it's been great fun for me, too.
My goal is to encourage creativity in both materials and subject. So far we've used watercolor paints, oil pastels, markers, crayons, colored pencils and lots of fine point Sharpies. We've been setting up at the kitchen table about once a week, during the little ones' naptime, and I usually have a theme of some sort. They are free to do something completely different but this helps if they can't "think of anything!". Some recent themes have been choosing and copying a creativity quote then decorating the page; a fairy watercolor suggestion from an Usborne art book I have; also, my mom sent the boys some photos from their visit so they scrapbooked a few on one of the pages.
Here's a photo of Trent's art journal page from last week:
For this page, we lightly taped index cards on a blank page (we used a double page spread; you are seeing the right side page). Then we sponge painted the page using our own selection of colors and way of painting. After the paint dried (it didn't take long at all since the sponges helped it go on thinly), we removed the index cards. Then we wrote something about our week in each blank spot. Trent chose to use 2 index cards on each page and then write one thing in each card, drawing a picture to go along with what he wrote about.