I bought these alligators (actually crocodiles) from US Toy Company for $1.48 for 12 (with no shipping cost).
This year I used spray adhesive to attach them to the card (other years I've used glue dots but I didn't have any at home). Very simple but very cute...especially because my kiddos love Dr. Jean's Good-Bye Friends song that we call See You Later Alligator. I also include a self-addressed note card, with the promise that I'll write back, to encourage them to write to me during the summer.
I'm linking up with 4th Grade Frolicc's Made it Monday Linky Party.