Justin and I did this for our wedding.  I think it was a great idea and wanted to share it with you too.  Instead of doing a traditional wedding favor, we made a donation to the Arbor Day Foundation in honor of our guests. We estimated the number of people attending our wedding, and donated the amount needed so at least one tree was planted for each guest.  With the donation, you get a certificate that you can display at your reception to let your guests know, you planted a tree in their honor.  Here is a link to learn more.