Okay! Has anyone played Set? Basically, the dealer lays down three rows of three cards, and the players stare at them. The first one to spot three that form a set calls out "Set!" and places those three cards in front of him/herself. The gaps in the card rows are filled with new cards; repeat. After the entire deck has been gone through, the player with the most cards in front of him/her wins. This works really well with collage cards, because you can spot all kinds of things that make three a set. Three red backgrounds, for instance, or three cards with faces, or three cards with words that begin with W. The categories are completely makeshift. Once, a player said, "it's a set because these three make me think of loss" and the cards had, like, a tomb, a postcard and a skeleton or something. Entirely up for grabs.
The other game I have played with them is also a create-your-own-categories game. It is like a version of go fish, again forming your own categories as you go. So you have a hand of five collage cards and ask, do you have any cards with rain? Or, do you have any cards with yellow in the foreground? Or whatever, and if not, you draw one from the stack and then discard. The winner is the first to lay down all five cards with a defendable category they ALL belong to.
I usually play to lose with my own cards, because I am so interested in what other people do with them.