Game jams will help you flex your creativity.
Playing other games, and as ComiNo suggested, alter some mechanics of those games. Change the perspective, swap turn-based for real-time and vice versa... Make the player play a different role. For example, I made a hunting simulator game where you play the deer!
Another source of inspiration is your debug mode. Because it let you discover if altering constrains leads to some fun. What happen if I have infinite ammo, if I can move trough walls, if time stops while I give order to my troops, if my car bounces on obstacles gaining a speed boost.