Lights? Lemme tell you about lights. Here's what we had in Britain in the '80s. (
1980's, smartypants.)
Four D Batteries in each of those things.
D batteries I'm tellin' ya! The pair of them weighed more than I did. Not only that but the front one bounced out of its bracket at the merest suggestion of a bump while both of them stopped working if you were foolish enough to ride with them in the rain. Having them on your bike, assuming they were working, meant you were in compliance with the law but that was the only practical purpose they served.
You kids with your fancy schmancy LED lights. You don't know you're born. (Waves fist at clouds and wonders once again why crotch is wet and warm.)
Anyway, seeing as you asked, my favorite doohickey would have to be my bike computer. All those years I didn't have any way of quantifying how much fun I was having.