Sending it to space (maybe our own star or one of the gas giants) would be very optimal indeed, but before space elevators it would be insanely expensive and thus impossible. And even with space elevators it would still be expensive.

Burying it very staunchly is the best bet. Sure thousands of years from now they might be dug up by people if we assume the civilization will collapse and rerise losing memories, but there could still be skull and other warning signs left. And it's not like there even existed other options. It sure is better than keeping them in barrels in temporary shelters, protected from rain but nothing else, which is still the situation currently practiced pretty much everywhere. That can't go on forever.