Last year when I was flying on April 1 I think I had a pretty good joke going. When we were about 15 out, I told all the pax that there was fog developing at our destination and that we would be holding, but that we had 3+ hours of extra fuel on board so we would be holding that long before we went somewhere else. Then about 30 seconds later, the captain came on and made the real announcement, saying april fools. Most of the pax actually appreciated it, although I don't know if I would try it again...