I also love getting "real mail" instead of just bills and junk. So I recently started randomly sending friends and family goofy cards just for the heck of it (of course it might be nice if I also remembered to send birthday cards but I usually forget) and it is so much fun to get a random card for no reason at all. I love the postcard idea!
My cousin recently used postcards for invitations, and then for thanks you's for her son's b-day party, with pics of her son.
(and I still have them hanging on the fridge since he's too damn cute!)
