That's the best part! It can be anything. An artists interpretation sent through the mail. A lot of mail artists do what you mentioned. Collage, painting, poems, photography, making your own artist stamps. Recycling, carving your own rubber stamps, using commercial stamps. Its all the things! However, there are people with special interests and they join groups to trade particular mail art or list it in their bio. Some like politics or animals or "obscure" things. I hope that helps. But whatever you like to do, don't be shy. Send some mail! I am always up for a trade of anything. I love it all!