Hmm. They would claw up wood. Dirt would get muddy when it rained, sand would get hot in the sun. Concrete would be ok, with some natural area. Wood chips (no cedar) may work, but be prepared to scoop them out when they're used as a litter box.
I'd say part concrete, wood chips, and a sand box (covered from sun and rain) if you want to encourage them to pee and poop outside too. With lots of perches and some large climbable logs for scratching. Grow some flower pots of catnip and cat grass outside the enclosure and trade off pots for them to munch on. But that's just my idea.