G.G.
Bree, thank you for considering donating these cards. I asked a very similar question on this site a few months ago and a mom suggested donating the items to Scottish Rite Hospital in Dallas (downtown, I think). The cards can be used by its patients as they work toward recovery.
I mailed my cards in and Scottish Rite quickly responded with a letter confirming receipt of the items.
You may need to call ahead and ask about those cards that have writing on the back but I'm sure they can use the blank ones.
Good luck :)