We also did some free painting. It might be a good medium to try some cards in, although I think we'll need smaller brushes.
Why don't you try it out? It's lovely to take a little time to create something beautiful, and seeing the kids work on their own art, reading all of the different posts sharing the ATCs, getting your own in the mail, maybe making a few new friends, it's all so fun! Kimberlee has links to previous swaps and ideas for inspiration.