Is NOT to sit down and rack your brain. In my recent experience, thinking too hard just ends up with some idea which you are not happy with. An idea that has been forced kicking and screaming into a miserable (and short lived) existence.

The best ideas come from the other direction. Try daydreaming, thinking of cool scenarios and beautiful shader effects (bloom, oh I do love thee). You are not trying to squeeze out a game idea, no. You are remembering all that you love about games, why you starting making them in the first place.