Here is a cute idea for the casual couple. Grab some blank tees in your favorite colors or match them to the theme wedding colors, add a message on them of thanks and attach each photo to a blank size-A6 postcard, and voila! ~ picture-perfect postcards.
Right after your engagement photo session, while you two are still looking good, put on your tees and ask a friend to snap a few digital photos of you two. Or have your engagement photographer take a few at the end of your session.