Ask HN: What to do with two unused domains?

I own collegetanks.com and everstretch.com. The plans I had for both of them fell through, so now they just sit on the interwebs, useful to noone - myself included.

My question is this: what can I do with the domains until they expire? Could I, for instance, put ads on them - for tank tops and stretching/workout equipment, respectively - and hope for the best? Or, could I sell them? Or, are there other options?