My sister Emily's getting married next year, and the wedding invite I designed includes a separate RSVP postcard that guests have to mail back. (I think it looks better than I'm making it sound, the invite. I think I'll post photos once they're printed.)
Taking a cue from comic book and magazine variant covers, I came up with two different postcard designs for my sister's wedding invite. They can give them out randomly, or, have one set of postcards for the groom's guests, the other for the bride's. Their choice. But I just like having a little variety.