* using public transport / not having drivers license: I got a driver's license only in my mid 30s and I really regret not doing it earlier. So many beautiful days wasted at home, lost touch with friends etc because getting out of town was too much trouble.

* related to that: staying at home too often instead of finding a SO. While I am very happy today, sometimes I wish I'd have found my wife and I had my children earlier. And maybe more children.

* procrastinating and eventually giving up on too many side projects. I have a history of starting awesome side projects and not finishing them for whatever reason, starting as a teenager. If I had finished even one of them, I may be able to live without a day job.