I use either paper towel or very finely ground sand that I actually buy from a breeder who lives locally. Paper towel for the little guys and sand for the big guys. I wouldn't personally use walnut shells because of impaction, or calcium sand for several reasons (a few of which being impaction, shed issues, and the smell lol, I really don't like the stuff..). I've heard tile is a good one to use, minus the, uh, smear factor, lmfao. I like paper towel because there is little to no risk of impaction, easy to clean, easy to see if babies or new arrivals are crapping normally etc. I like sand becaue it looks attractive, easy to clean with a scooper, and the dragons can dig a bit, which I find the girls especially really love.
Those pictures are so old now. My little red lady is much bigger, haha. If I get a camera for Xmas I'll have to snap some new shots of the group

Dragons rock!