Inspired by some of my tweeps, I spent $1.25 on some modeling clay, dug up my backyard, and went to town.
There's also some activated charcoal between the bottom gravel layer and the potting soil top layer for odour control. That stuff's expensive! I had some cedar sawdust on hand from my cedar planter project this summer, and I sprinkled some in to smell nice (mmm ... cedar) and to hopefully keep the nasties at bay. All in all, a fun couple of hours!