If you've come all the way here, I'd love to hear from you. Where did you find the card? What did you do with it? What have other people done with it?
Just drop an email to firstname.lastname@example.org and I'll put anything you want to say on the card tracker. Let me know what name to credit you with, and any website you want me to link to. Photos or (even better) high resolution scans are brilliant, and I'll listen to any feedback you have too.
Scott McCloud defines art as "Anything not directly linked to our basic instincts to survive and/or reproduce." Anything you choose to add to the card is art, and far be it from me to cramp your style. If you're looking for advice however: I'd recommend avoiding anything obscene, and try to interact with the existing art without completely filling the space. That way, there's space for the next person to add their bit and continue the card's journey.
When you're done with the card, you might decide to move it on. It's entirely your decision as to what you do with it, but please consider the following: Anything outdoors will be exposed to the elements, and the card may not survive. Try to avoid anywhere where it'll inconvenience people, we don't want to be littering. Finally, try to put it somewhere where it'll be discovered eventually, but not immediately. An obscure place can heightens the sense of adventure and discovery.