Black flourite sand is one of my favorite inert type sediments, but the color issue is more aesthetics really.
Water falls make a lot of racket, unless they are small, then they do not look as good , but if you work at it, you can still pull it off well without too much noise.
A cascade might be interesting also.
Lava is very light weight.
Making your own styrofoam background and scape might be a wise option.
Takes some Crafts and DIY skill, but the pay off is really good.