How about recycling it?

Earthworks has an easy way for the guests at your next event to recycle their PVC plastic hotel keys. Earthworks takes items such as these (and other plastic items such as gift cards, ID cards, signs, point of purchase displays, membership cards, bookmarks, rulers, hang tags, drivers licenses, circle calculators, playing cards, luggage tags, and specialty advertising) and turns them into sheets that new cards can be made of.

You can either ask your hotel to gather them all up for you, or you can drop the cards in the mail yourself. Address them to Earthworks, c/o Halprin Industries, 25840 Miles Road, Bedford, OH 44146. Magnetic strips are okay. Just don’t send credit cards or those that contain paper backing