After you find a few cartoons you like, print out a few of them.
Get those cute decorative scissors that make the trim 'fancy'....
Cut each 'strip' - have the kids write letters to grandma & grandpa - aunts - uncles; friends, etc. Let them write a quick note on the BACK of the strips you've cut - maybe put a little glitter on the strip - tuck it in with your letter, or let them send their own.
Get out your newspaper; clip a local human-interest story - something that you think gramps would like - slip that in too!
Throw in a few of the very large paper clips - those older folks tend to buy those tiny ones, and they never work as good. Have the kids sort out all the red, white, and blue clips - send to someone who's in the armed-forces; a nice thought....
Decorate the envelope with thumb and finger-prints of the children - add yours as well. If they can finger-print us at the bank and we don't mind that ink all that much, a little ink in a pretty color would be a nice way to 'personalize' the envelope (and cover up any coffee stains at the same time).
Now it's time for Calvin - here's your link: http://www.marcellosendos.ch/comics/ch/
