Here are some random ideas: "morning in the sun", "rainy afternoon", "scary night", "storm", "time", "secret place", "a walk in the forest", "space adventure"... I don't know, things that tell a story and enable the imagination to go wild!!! It could also be books or songs titles (and it doesn't even matter if people don't know the song or book); colors would work well too I think. It could also be more abstract or philosophic concepts, like "freedom", "reality", "the theory of relativity", or whatever. OK, I'll stop here, now everyone's going to think I'm totally crazy!!!