What I'm looking to do is create a type of 'ground' like rock or earth, etc etc. I'll probably just use a printout for the background but I'd like the ground to be more 3-d since they'll be standing directly on it.
The best thing to use is the real thing. I've made dioramas in the past and used sandbox sand from TRU to cover the ground. Just take some foamcore or whatever you're using for the base and cover it with a thin coat of white Elmer's glue. Sprinkle some sand on, be generous, and wait about an hour. Then just tilt the base and all the loose sand will come off, back into the bag. Then, you can repeat on any areas that you may want to add more onto, or any areas that may have been missed.
Go to the park and pick up some rocks. You can put them in the oven on a low temperature just to "cure" them, to kill anything that might be living on them or in them. I used to do this with rocks when I was keeping various lizards as pets some years ago.
You can go to a pet store and pick up aquarium plants in many different shapes, sizes, and colors to match just about any look you are going for. And since they are plastic, you can always paint them if they aren't the correct color. You can also pick up sand in different colors at the pet shop, if you were looking to make a more "alien" enviornment, that maybe doesn't have normal earth-looking sand. You can also pick up pieces of driftwood that come in all kinds of shapes and sizes.
If you have a hobby center or train shop near you, there are plenty of options for making fake rocks and scenery. Or you can order supplies like that online. One place that I have ordered from in the past is Tower Hobbies
. Good prices from what I've seen, and quick shipping. They have plenty of great stuff that can be used, like grasses, earth and ground cover
and scenery molds and accessories
Good luck, and make sure to let us know how it turns out