First design project was a touch-screen drive-thru ordering system. Why have someone inside the restaurant, who makes minimum wage, push a screen when we tell them what we want? Why not just touch the doggone screen for yourself and order exactly what we want? Nothing gets lost in "translation" and we won't have to listen to a crackly headphone microphone wearing person. Intuitive user-interface. No crackling voice. Pictures displayed next to what you're ordering. By the way, this was before touch screens so I had little buttons to the side of the screen like they do at ATM's. I was not yet a teenager, and multimedia wasn't a big thing yet, so my design was all on paper.
Built and installed a "remote bedroom door lock" so I could lock and unlock my bedroom door while sitting at my desk. I had few interesting friends so I did my homework, didn't want my brother or parent walking in, and didn't want to get up to lock/unlock the door. The thing had two strings...one for lock...one for unlock...and a system of eyelets, a couple levers, a latch, and some paper taped to the thing so I could keep track of which string locked and which unlocked. Then ran the strings through more eyelets along to the far wall, where they then dropped down like window blind strings. I was barely a teenager. I thought it was cool and neat to have designed something like that without ever have taken engineering classes.
Built shelves, Ikea furniture (aren't those instructions from another world?), fixed bicycles, mentally redesigned major parts of car engines, prototype a hydroelectric generator (more on that to come).