I've been in this position, and I've concluded: if you really have no financial constraints, you'd be daft to spend your time working on a startup, or solving the dull, pedestrian problems that are of such utilitarian concern to society.
If you're creative, why wouldn't you just be an artist and make things that are personally and intensely meaningful to you (but only to you)?
If you're an engineer, why wouldn't you just spend your time inventing new esoteric programming languages to implement Conway's game of life, rather than the drudgery of, say, making and debugging a robust automated toll booth, that could save society millions of dollars? That only makes sense if you're anticipating an eventual return to the monetized (wage) economy.
Or - start a family and invest your full time into raising and educating your own children - probably the most rational choice, with the least value to the rest of society.