If you do some basic testing with vinegar to see if your rocks will dissolve in your tank, you should be able to clean up rocks and use them. There seems to be two schools of thought, boiling them (though this can possibly be risky, as any gas pockets in a rock could expand with the head and explode the rock, there's a good thread around here somewhere about it if you use the search function), or soak them in a bleach solution for a day or two and then soak them in some water heavily dosed with dechlorinator, possibly even switching out water for fresh dechlorinator a few times to be safe before adding them to your tank. I've also heard of people running them in a dishwasher (without the soap and drying solution stuff), but don't personally know how well that would work.
Why do you want to drill the rocks, out of curiosity?
As for "other things," I think as long as it isn't a resin-y wood (such as pine) you should be ok to find driftwood, though of course harder woods don't decay as quickly in the tank, and if it's from saltwater you maybe need to make sure it's not full of salt? I've not done that myself, though, so DON'T take my work for gospel. I've also thought about harvesting aquatic plants from local waters, but have never done that either, and don't really know how feasible it is--though I've heard of people doing it.
