What do you guys think? Could I use something like large, smooth rocks or flat marbles as substrate and get away with it? My bet is it's a clean alternative to aspen, plus it's reusable. I only have to worry that they wouldn't be comfy, but i could put some softer stuff down in the hide area and give plenty of places to climb (every ball i've had who has had the chance to climb has seemed to really enjoy doing so).

I eagerly await opinions!