if you own you own business you can make a wall of business cards and then when people come in they will want to add their cards to your wall (maybe you could charge) and then you could have the biggest wall of cards and then you would get into the guiness book or on the travel channel or something
old cards of any kind are perfect for decoupage- you can decoupage anything- there are many websites explaining the process- just winzy search decoupage and you will have plenty of ideas.