I'm not quite sure what the proper method is to engage your child in the act of sending out thank you cards but I found after Hen's birthday that using leftover blank postcards and giving him free reign with paint worked rather well. What was so awesome about this experiment was that after he completed painting each postcard, he would say "fish" or "turtle" or "jellyfish" because that is, apparently, what he drew. Kids are cool.
I just found the scrap of paper I wrote the information on. Reading from left to right and starting at the top we have: a fish, a sea lion, a stingray, a sea turtle, a jellyfish, and another stingray.