Card making is a simple craft that can be done with just a few stamps, a black ink pad, markers, pretty paper, and your imagination. If you stock up on stamps that cover all of the occasions in life (birthdays, holidays, get well, sympathy, etc), then you can whip out a card in no time. The beauty is it saves you from running to the store to make that last minute purchase.
Still not convinced? Try starting with a card making kit. This will include all of the supplies you'll need and explicit instructions on how to make a set of themed cards (such as Happy Birthday or Christmas). It will give you a real feel for what card making is all about. I recently did one on Christmas (as seen in the photo below) and had a blast.