I love trying different background techniques for ATCs. Because they are such a small platform to work on, you can really commit to trying some crazy, different backgrounds and you haven't invested too much product into it. If you do happen to stumble across a technique you like or a background you like, you can then incorporate that into a larger collage or mixed media piece.
For the backgrounds on these three ATCs I used Polished Pigments from LuminArte in a couple different green's and blues mixed with some liquid medium. I then went around the edges with Distress Crackle Paint in Picket Fence. Once that all dried, I went over it with some Diamond Glaze so it wouldn't flake off. I then used some copper acrylic paint and bubble wrap to make some random impressions. I was very happy with the outcome of this background and I fully intend on doing it on a larger scale.