In the past three days, I think I've pretty much hit on all of those hobbies. Hooray!
My recent foray into the nearby vintage store led me to some fun old postcards. There's one from Guatemala that I plan to send to my friend Amy because we traveled there together, and one that will go to Matt for the same reason (but it's of Carlsbad Caverns). Then there's the Texas Longhorn Cattle just because I live in Texas now and I may need to send the unexpected postcard. I purposefully bought ones that haven't yet been written on, so I could really use them.
It gave me the idea that vintage postcards would make great Save the Dates (my friend, Camella, also used them for her RSVP cards). You could just print your message on clear labels or colored sticker paper that can be cut into labels.
I tried to do a quick search for others who had gone this route, but all I could find was this fake vintage postcard.