I use a fairly new song by Jarion Hamm entitled "We Enter Your Gates". He is the minister of music at Bishop Dennis Leonard's church in Denver and a personal friend.
I do that song because it has a good mix of scales, unison and harmony.
It's also a song with a good message that will help them to focus on rehearsal and not on what they went through to GET to rehearsal.
If you would like to hear the song, I have the file saved on my computer.