OK, here we go.
Saran wrap on the ladies toilet. Also, a rubber mouse opposite the loo that isn't seen until the sister has her pants down and is sitting.
A pencil eraser with a notch cut in it inserted into the drinking fountain. Shoots water up the nose.
Make a "U" yoke with a big paper clip. Stretch a rubber band across with a small paper clip in the middle and wind it up, wrap in a piece of paper. Before the meeting plase this packet on the floor in front of a seat near the middle of the hall. As it's unwrapped by the innocent it gives a loud buzz. (Even better, put confetti in the packet too)
If you've got the nerve, stick a panty liner to the bottom of your shoe, then set down on the isle and cross your leg. (red coloring optional)
This is only a sample if you want more let me know