I've been working on chipping the paint off the rocks by the creek with some good results.
Although chemical paint removers would work (I tried it), I found that since the rocks had a somewhat flaky outer layer, a rock or hammer would chip off the outer layer of granite without too much trouble, thus the paint too, leaving the rocks looking fairly natural in the process with no toxic fumes and mop up to worry about. So far, I've chipped away about 1/3rd of the dark green outline paint, spending less than 2 hours on it so far. The heat has kept me from returning to that project for the last 3 weekends though. Rick and Greg deserve some credit for helping to prove the concept.
Unfortunately, the graffiti on the Goat Trail by the split rock can't be chipped off, as the granite there is a more polished rock - not flaky like the other rocks. I'll probably use chemicals on that one.