It is easy to build floats. In just a couple of hours a group of refugee kids build a float for the performance "Clothes make the man" by Helle Fuglsang. Then sailed the floats through the canals of Copenhagen. I instructed the kids in the building process, but
A little experiment getting smd leds to shine in water using 230 V. The water is conductive enough to have the voltage drop over the smd led make them shine. A real "Don't try this at home" experiment.
Converting an old fire extinguisher to a flame thrower. A fiend wanted to give his kid a cool birthday present, so I made this for him. Probably Not-Safe-For-Kids.
Ritually getting rid of your old electronics. Electrocute it and burn it.
To pick up some fairly heavy anchors, I designed and built a small sailing crane. Worked quite well.