Anything with a lot of surface area. Ceramic rings, bio-balls, plastic pot scrubbers, whatever.
But you need to think about why you're picking any bio media at all. Why not just go with mechanical media? It all doubles as bio media in any case. A simple sponge is both mechanical and biological media, while a plastic bio-ball is just biological. Just using mech media increases the amount of time your can will last between cleanings, and makes your can even more efficient at physically cleaning water.
If you have a good reason for adding strictly biological media (other than because it's what lots of people do), I recommend dollar-store plastic pot scrubbers. If not, I'd say just go with filter floss, sponges, or something along those lines.