Playing other people's games (particularly indie and amateur games), looking through my old art and pencil + paper RPG folders, jotting down ideas on paper and drawing concept art.
Also, for dungeon ideas I used to ask people "If you could explore any environment, what would it be?". I've gotten all sorts of answers.
EDIT:Drawing is probably the best way to amass creativity. All of my good ideas (most of which I never put into a computer game) came to me when I was actively drawing cartoons. Check out Mark Kistler's website:
http://www.draw3d.com/