I have my own ideas on how to do this but I think your looking for something a little less destructive. Now that you've got your guns all lubed up have you done some serious target shooting? Is it repeatable enough to pull this off? Try to set up some stationary targets and see how easy they are to hit. It was my recollection that the gun will not lower enough to hit a ground target at less then about 20 feet, at that point a hard shot.
So the flag idea could work nicely. A simple frame with maybe a paper towel of light paper stretched across it.

Another cool option might be to set up bunker or tank or some target and see who can be the first to hit it from a minimum distance. You could have rules like 3 shots then you have to move a minimum of 3 feet sideways or back, never crossing a 'foul' line like in bowling. For club fun you could make a cool bunker with simple hinged plates that could activate something when they are hit like a smoke generator or explosion sound, or a real explosion. (Sorry about that last one I forgot, less destructive)It would be like when you throw a ball at the target and the guy in the dunk tank falls.