My understanding of an informance is that it represents what you happen to be working on in class. For example, the PTO asked me to throw something together for an event next month. They want each grade level to be featured for 15-20 minutes, to get the parents to come to the area where the silent auction items will be. Perfect opportunity for an informance.

K - Students sit in several long lines. The person at the front of each line plays a drum pattern to a Sousa march. Every 16 counts, I play finger cymbals as their cue to march to the back of the line while everyone else scoots up. (They would do this for HOURS!)
1st - I read "Mortimer" while they provide accompaniment and sound effects with unpitched percussion and xylopipes.
2nd - Mexican Hat Dance and a dance I made up for "Old Joe Clark."
3rd - Vocal warm-ups from Music K-8 and "Alouette" from our text.
4th - Recorder Play Down: Everyone stands to play the music which is projected on the large screen on stage. As they reach their highest belt, they sit. We keep going until only a few are standing.
5th - Drum circle
My recorder ensemble and my swing dance group will also perform.

I chose these activities because I can do them easily, regardless of how many kids show up. All I have to do is haul some stuff down to the cafeteria and have fun. It will take 2 1/2 hours, but it will be fun.