I love drawing on odd sized pieces of paper. The various shapes and sizes of paper force you to create is spaces you're not used to creating in. Drawing then becomes a bit of a puzzle to solve.
I found a stack of unused cards in my office and couldn't resist a few doodles. I drew a sweater wearing dragon, a mushroom villian and a ninjabot. Lots of ninja's this week.
Try it yourself! Go out and decorate some envelopes and fill them with letters to family and friends. They'll love them and you will have made something unique!