Replace your missing bulb with an energy-saving bulb. Make a personal pledge to serve others by pursuing a more sustainable way of life.
Pledge made. One thing I've discovered while doing this has been that I do quite a lot of these things already, without even noticing. Little things really do make a difference, and they're quite easy to do.
Now I just have to read back and see what thigns are still outstanding, count up my forfeits, and send a cheque to TearFund.