[M_Six]

A colleague of mine and I were contemplating a start up business back in the early 90's. I borrowed $10k from my folks for it, but then we never did get around to starting things, so I paid my folks back. If I had taken that $10k and bought Apple stock and let the dividends buy more stock every year, I'd be filthy rich now.

To your original question, I'd invent a tri-corder type device that could be used to scan your entire body for things like cancer or other issues while they were still treatable/correctable.