Public transport! Hop on a bus for an hour or two. Change of scenery plus being trapped in a bus without many distractions is great for thinking.

I like to people watch -- riding public transit is like accelerated people watching -- while looking out for people that seem to be enjoying themselves with unstructured play, whether that's walking in a special way, toying with an object, or what-have-you. Humans like to make up meaningless goals just for the fun of achieving something hard; games are play where a part of play (inventing goals) is outsourced to an expert :)

Even if the play activity itself can't reasonably be gamified, I like to ask myself "Why is that fun? What's rewarding about it? Is it a kind of fun we see in a lot of existing games, or a kind of fun we rarely see in games?"

I carry a small pad or some paper and a pen on me at all times so that any idea I get I can write down. Then when I get home I do two things, rewrite it and flesh it out in a notebook and then type it up in an ideas file. I started doing this back in 1996 or so, so I have a lot of backup ideas (just a matter of finding time to make any of them). I tried to start an RPG back in June 2002, but as of yet it is just all in my head and on paper.