Preferably any tile that has a somewhat rough surface to it. The sandstone sounds good. Slate is another good one. Just any tile that is not smooth(because it is so hard for them to walk on). I have slate in mine and I love the look.
you can grout it and have it permanent and just wash it out with 5% bleach to 95% water mixture, or you can have the tile lay in there loose, and just remove them for cleaning