I’m not sure what the appeal of solo play is, because roleplaying is all about building collaborative stories. If you’re doing it solo, you might as well just write a novel (also very cool). I guess you could use some kind of random tables or something to help you come up with material when you’re stuck with writer’s block, but that’s about all I can see it being useful for.
Anyway, a fun, simple, and free (at least in the sense that you can acquire it without paying money, not in the FOSS-type sense that you are free to modify and distribute it legally yourself) little system is Risus.
Board games have a predefined goal. They aren’t designed around collaborative, open-ended storytelling like roleplaying games are.
Risus is quick and simple and can be used for any kind of story and setting you want to build a silly little adventure in. Good for when you want to play something with folks and not have a lot of preparation or hassle.