learning to play guitar... my parents encouraged me from a very early age but i kept resisting it. both my brothers could play and i just got frustrated and gave up too easily. the summer i discovered the pixies, that changed! if i'd have learnt as a kid i'd be awesome.
I think there's a few things writing more instead of just thinking about writing, starting a band just for fun and not really to play for any audience, travel more, volunteer for several months in an area facing a famine or some kind of natural disaster.