Most soldering irons are either on or off; it would be much more useful if you were able to change the temperature of the iron. So I designed a simple control circuit that will let you adjust the output of a heating element.
A great transition from simple circuits to solderless breadboards, the springboard is a 50-year-old relic that’s difficult to find commercially these days, but easy and cheap to make.
A few times a year I volunteer to an arts organization and serve as their “house sound and light guy.” I usually run sound and do the lights for the traveling acts if they don’t have a tech with them.
Cartesian diver toy "Diving Tony" was a cereal box favorite. You can make one quickly from just a few parts. Use clear PETG tubing and a piece of solid styrene to make the diver follow your commands.