When I bought my current house it came with an abused Vermont Casting Defiant stove, it had been over fired (over heated) for many years and nearly everything was warped or crumbling. The frame and its enamel still looked great and I didn't know just how bad it was so I started gutting it and buying parts. The more I took it apart the more I found wrong, but I was determined. Eventually I succeeded but the parts alone cost more than the stove was probably worth but now I know every inch of stove, how it functions and I have everything dialed in perfectly. I'm very happy with how it works now and it keeps our home warm all winter long with very little natural gas.
I found this power wheels on the side of the road, abandoned. It was in rough shape but I wanted to heavily modify it so it was the perfect start. I started by modifying it to run off the 18V Bosch drill batteries I had around, then added a fruit crate to the back so it was now a pickup truck. My daughter adores it and drives it around all summer and tends the flowers and garden with it. I even attached a 1 gallon tank to it so she can fill up, drive around and fill her watering can near the flowers. To date I haven't spent a dollar on this adventure, it was built entirely from junk I had in my basement because I'm a pack rat.
For some fun with my nephew I made a bottle rocket launcher. It's a base with a lock, you place the bottle with some water in the lock, pump it up with a bike pump then pull the string to release the lock and shoot the rocket. Lots of fun!