If you plan on sending your save the date cards in February, or plan on having your wedding near Valentine's Day, I was thinking that those boxed kids' valentines that are sold at stores like Target and CVS would be cute (and inexpensive--and after Wednesday even cheaper) save the date cards. What sweeter way to celebrate your wedding than with the theme of candy pink love? Especially in the form of Snoopy, Hello Kitty, Spongebob Squarepants... dozens of your favorite characters to choose from.
You'd have to write your save the date message on each one because they're too small to go through the printer. You have to buy new envelopes if they're being mailed (all craft stores, and the stationery aisle of some drug stores, sell plain note-card sized envelopes).
They could also be used as placecards at a Valentine's Day event. They are informal, but they are unique and whimsical. They might be just what you need to set the tone for your wedding and your wedding planning.