After seeing this on numerous sites (I think I first saw it over at Karen's), we started playing this fun vocabulary game over the weekend. Finally a "time-waster" I can get behind and not just because of the donation of food to the poor, but because it's well-designed to improve vocabulary. They test you with a few beginning words to get you to a point where you're being constantly challenged. It seems to work well!
UPDATE: We're also finding this travel/geography game to be a lot of fun.