Music practice. They didn’t push me to do it and because I was quite a naturally lazy child, I wasted my musical talent (I was pretty good at it but didn’t have the discipline to practice). By the time I realised I wanted to, I was a bit too old (17) to catch up.
I had a friend at primary school whose very strict father made her get up at six AM every day to do 30 mins practice each on two instruments before getting ready for school. I was pretty horrified at that, but somewhere in the middle of the two extremes would have been good! I now try to make sure DD does enough to keep up her music, which again she’s pretty decent at though not a prodigy. So far she has got to grade 5 with distinctions just through ten minutes practice most days. I wish I had known as a child that little and often works fine: and that I didn’t need to put in huge amounts of effort, just enough effort every day to keep improving.
In general my parents probably let us watch too much tv and not really stick at hobbies. I did ballet, guides, and the occasional other thing, but I could have done with them setting a few more boundaries about going and doing things. If (like all kids) it was easier or more comfy for us not to go to an activity, or cold/rainy and TV was more fun, they just let us not do things, which was not a good habit for later life.
I also wish my parents had told me how talented I was and made me develop my talents. Clearing some stuff out in their house, I found drawings that I’d done, poems and stories that I’d written, and realised that I was really talented, especially at drawing, but nobody ever told me! They just said “that’s nice, dear”, but never actually explained to me that I had genuine talent. I had no idea and dropped art and stopped doing anything arty because I thought I was no good at it — which couldn’t have been further from the truth.