We go to thrift stores and buy electric kettles, irons, computers, hair dryers, bike pumps, clocks, toasters, and coffee makers to disassemble and reassemble. I've also bought inexpensive new electric toothbrushes and have been known to take apart the vacuum.
"How it's made" is my son's favourite show, and you can find 5-6 minute segments on many topics on YouTube.
Anytime we need to do household repairs or installations, we ensure DS does as much work as possible. We did a simple woodworking project together recently to build a toy dump truck, which he also enjoyed.
We also do a lot of science play at home, making things like anemometers, hovercrafts, thermometers, etc.
