I wouldn't use pebbles. If you want something easy, just go with paper towels or newspaper. You can always get unprinted paper or butcher paper if you want something "nicer" looking.
In aquariums the fish don't actually live on the substrate, i don't think a snake would find it comfortable.
Waterfalls are bad. They are just a breeding ground for bacteria and there have been quite a few threads on peoples snakes getting STUCK inside the compartments. Snakes will inevitably defecate in there and it gets nasty quick.
Try a larger water bowl under the heat source, or switch to a tub if maintaining humidity is too hard.
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