I decided to incorporate a mail swap when we learned the song Holding Hands Around the World. I found two Primaries in Italy and Japan to do our mail swap with (our Utah Ward was big enough to split with two other Wards). Finding a Ward turned out to be trickier than I thought, but I pulled it off. So, if you want to do this be sure to plan ahead. Way ahead. This activity is not one you can just pull out of a last-minute bucket. It takes some serious planning. Another thing that really helped was having a template for the kids to fill out. I also hung one I had already filled out on the board as an example. I had a younger version of the temple for JR and an older version for SR.
Here is a sample of some of our letters: