If you can find some chunks of pumice (I think it's usually marketed as 'feather rock'), that's pretty easy to cut/drill/carve up. Just wear gloves and a mask, and try to keep the rock wet.
You could carve up caves, or even little rock planters pretty easily. Only problem is that pumice often floats, so you'd have to silicone it to the tank or something.
I've seen People make caves and such out of chunks of lava rock, but that's a lot more difficult to work with.
Or you could try an epoxy/sand mix and molding your own, or coating something else with it.
Take a look at what some people have done to make paludarium backgrounds. They usually have pockets for planting, so you could do pretty much the same thing, except all underwater, and have the pockets just be raised beds or something.