Boating/astronomy/skydiving/diving... take your money pit of choice the bank manager might take a dim view of them all.
Still, not speaking for others, but astronomy has been the most rewarding for me in terms of return and personal growth, i cant put a price on it.
mostly 'cause we can share it with almost anybody and they are usually captivated by the deep night sky, what a great reward.
keep on spending...LOL