Here's a nice little show-off piece for your choir! This "voccata" (a made-up word meaning "vocal toccata") is optionally 3-part or 2-part, as you prefer. Part 3, written in bass clef for your lower, changing, or changed voices, is very basic and expendable.
The beauty of this piece is that it is also an excellent workout. If you just accompany it live, or if you use it a cappella, you can keep speeding it up to challenge singers more and more. For maximum versatility, the kit includes isolated rehearsal parts, a cappella tracks, and tracks in various tempos. (from Music K-8, Vol. 22, No. 1)