If you really, really, don't care about getting from one place in space to another quickly, a solar sail can be a good option. A solar sail is light (no pun intended) and requires no onboard fuel. At first glance, it sounds like an aerospace engineer's dream. The big issue, though, is that it takes a long time to accelerate to meaningful speeds. The problem can be tamed a little by making the solar sail bigger, but that means that it can become unwieldy (and thus harder to deploy), and a little hard to stuff into a rocket. So once again, here I go with a "size" question:
How big can solar sails reasonably become (assuming finite resources)? What are the biggest ones currently on the drawing board?
Finally, as called2voyage has pointed out, you could make a very large solar sail with "an asteroid and a 3-D printer." Is space-based manufacturing a reasonable option?