Instructions
Find a neighborhood child who would appreciate the cards as a baseball fan or a collector's item, or simply as toys. Allow the child to have them for free, or sell them in a garage sale.
Donate the cards to a child's shelter or a school. Like the last step, this may get the cards into the hands of somebody who will appreciate them anew.
List the cards on aution sites like eBay or a site like Craigslist.com to sell them or donate them to a collector or somebody who is looking for the cards.
Use the cards as pieces of scrap paper. Write notes on them or leave them around the house for shopping lists or bookmarks.
Place them in your recycle bin with other paper products so that they can be recycled and reused in other paper forms.