If you're concerned about keeping the water clean, use a ceramic bowl that is smooth and easy to wash. Imagine what would happen to the waterfall thing if the snake pooped in it!
If you are daydreaming about an attractive, enriched enclosure, check out this Tumblr: http://rainbowsnakes.tumblr.com/snakesetups
They have some really nice looking setups, plus good information on why the stuff in them is appropriate for the different species they keep.
Sand isn't a good choice, but there are lots of other substrate options that are appropriate and it's perfectly fine to use different types in different areas of the cage.
One thing to factor in, if you want to use live plants, is lighting. Plants will need full-spectrum light, which should be on a timer for convenience. It won't bother the snake to have it on for 12 hours a day, that's a normal photoperiod anyway.
I'd also go with a radiant heat panel instead of a red light bulb for heating; the RHP can be mounted to the inside of the ceiling and won't get hot enough to burn on contact like a light bulb will. A light bulb either needs to be mounted above the cage and shining through a screen, which makes humidity more difficult because of the open screen, or inside the cage and surrounded by a mesh guard so the snake can't touch it.
