I was actually pretty sure this exists, but much to my surprise, I couldn't find it on the Net, so I'm posting it here.
My proposal is the "Mood Fan", a simple hand fan with a smiling mouth on one side and a frown on the other. The imagery I have in mind is mostly humorous - two people talking on a hot day, one person telling something and the other one expressing his emotions simply by flipping his fan from one side to the other.
With a bit of inventiveness, I'm sure it would be possible to accomodate more than just two moods, but maybe the black/white, yin/yang approach already does the trick.
Reward: A set of these fans for me and some of the people I know