Any postcards are great, I've enjoyed all the cards I've received.
If you are able to send something with cartoon characters or animated or bawdy or racy or macabre those are my favorites. Orange is my favorite color of all time! Also anything Moomin or Muppet or Doctor Who makes me smile.
Tell me about your favorite movie, or send the postcard with a description of your day, or a story or joke that you like to tell. Maybe just tell me something you love about where you live.