You need to let them take apart a small appliance, like a toaster or an old vacuum cleaner. They don't need to repair it, just take it apart. (If your husband isn't handy, go buy your boy some tools. Screwdrivers and socket sets will take apart and reassemble anything small.)
Boys have spatial tools that girls don't have and they get very VERY absorbed in what they are doing. My eldest took apart my mother's back porch screen and her patio furniture with a screwdriver she gave him to play with when he was three.
Since I have three sons and three brothers, I think the best advice is to remember that they are BOYS and not to freak out when they decide to beat pop-gun caps with a hammer or jump off the roof--anything over the first floor is verboten on the last part.
I love hitting caps with a hammer. We used to sit in the street with a roll of caps and bang away. One day one of us got the bright idea to wack the entire roll as we had several boxes of caps and felt quite rich. My friend had a two-pound sledge and hit the roll really square. The consussion and the flash was more than we had planned on. Thinking back now I am grateful and lucky we did not have an anvil, a six-pounder, and a whole box of caps.
Actually, my daughter is the one that likes to build things. We just moved to a new house. My husband took down our old swingset, and my daughter was the one that helped the whole time. My son and my other daughter got bored helping.
I'm one of those girls that does have spatial handling. I have a degree in engineering, so I'm pretty handy.