Yesterday while walking to a nearby coney island for lunch, I saw a man flying a kite, and thought "what if I were to hang an antenna from there?". I thought it seemed like a stupid idea, but why not? It would give it some elevation with the most minimal of costs. My only real concern is whether or not it'd be possible, or even just likely, to get the kite to stay up for an extended period of time - it's already dependent on a wind after all. However, this guy seems to have put some time into it, so maybe it's not as absurd as I had thought. He even talks about using balloons, which should be even better - lift isn't dependent on wind anymore, so even on a completely still day it could be used.
I'm thinking about trying one or both of these this summer, just for fun.