I love, love, love postcards that have "Greetings From. . ." and then the place.
Postcards of cheesy touristy places, or just plain fun postcards work as well!
Other postcards I enjoy receiving:
-UNESCO World Heritage Sites
Any postcard is appreciated, so if you don't have any of the above, no worries!
If you speak/write another language and can write something with the english translation, that will be great! And please include the date on your card!