"Finding the key" is also a good exercise for a cappella groups, if the singers are talented enough to pull it off successfully.

The general idea is to select a song, then have each singer pick a note to start on that is fairly close to his/her actual note, but is not the correct one. Then, the group starts singing and attempts to go from horrendously off-key to the correct key (or something close enough) in the space of a measure or two.

If executed properly, knowing how to do this helps your group in the event that all hell breaks loose during a show and you get badly off-key in live action. A subtle gesture from the music director can cue this in.