Reptile carpet, paper towels, slate tile, roll-out shelf liner. These are all great options. Personally, I like slate tile the best because it looks nice, is easy to clean, and will help wear down the nails a little bit.
You may just need to spray the enclosure a little more to keep the humidity up. But, that'll be the case with just about any substrate that you'll choose.
You could technically choose coconut coir (bed-a-best or the compressed dirt blocks), as this will help with the humidity, but you'll have to worry about ingestion, which can be fatal if the signs aren't noticed and the intestines get clogged.