Legos!
I have an older brother, so I had access to Lego Technic bricks from a very early age. All I ever wished for as a child were Lego sets. Birthdays, new year's eves, ends of schoolyears, always Legos. Sometimes my parents would join forces with grandparents, aunts, etc. and get a
really sweet set. Man, those were the days. I would follow the instructions, built the model once and then tear it down the next day, because I needed the parts for something else. I could easily spend hours just tinkering and building stuff. Everyone thought I'd end up being a mechanical engineer.
I remember this one time, my brother told me that we should both build Lego guns and then we could play soldiers. He finished his gun much sooner than I did. He made a shotgun model with a movable reload mechanism. I kept building and he kept nagging when we are going to play. When I finally finished my build several hours later, it was a real thing of beauty. I had a pin attached to a trigger, which held in place a small brick. I stretched a long elastic band and fixed it behind that small brick. When the trigger was pulled (much like a real gun), the pin would release that small brick and the elastic would fling it down the barrel, shooting it out of the gun with force. For some strange reason, my brother suddenly didn't feel like playing soldiers anymore.
I think I was 8 or 9 when I got my first Lego Mindstorms kit (bought it myself!) and at that's what shifted my interests from mechanical engineering towards electronics engineering.
If my kids (whenever that might happen) show an interest in Legos, they are going to get Legos. Lots of them. Daddy
might tag along, you know, because someone has to show them how things should be built.