It's best to use something solid. For visually appealing options, slate tile or excavator clay (hardens when it dries) works well. I've also seen people make elaborate landscapes by carving styrofoam and covering it with grout/other substances that look good. Less visually appealing options are paper towels, reptile carpet. There are other options, too. People get quite creative making safe and beautiful set ups.
Gorgeous looking leos and awesome habitat. I too would recommend against the sand or at the very least make sure you're using clean playbox sand from Home Depot and not calcium sand from the pet store. Leos can snarf it down and hurt themselves. The habitat really is stunning!