Nothing? It's really easy to clean. You can drill holes through the bottom and put drainage right under it for easy water management.
If the bottom is unattractive, go to an aquarium supply place and pick up "liner". There are loads of patterns, including things like rocks. There are solid colors. It'll lay down flat on the bottom, you can drill right through it, it is plastic so you can wipe it off as needed and the water won't be a problem.