I painted these cards a couple years back, for family and friends. I used watercolors and watercolor paper cards. You can sometimes buy them at Art supply stores.
Painting is always nice, and painting on other surfaces besides paper and canvas is nice also. All you need is some nice smooth rock or wood and voila you have a painting surface. What is wonderful about painting rocks and wood pieces is that sometimes the shapes are made for you, you paint what you see.
I too have dabbled with painting rocks when I was younger and again recently while painting with some kids. Painting rocks, if fun and fairly cheap if you have rocks around, and paint already.