JJ Watt -- I Know How to Spend My Fortune ... MAN-CATION!!!

You know when you're a kid and you promise your friends all sorts of cool stuff if you ever get super rich?

Well, J.J. Watt is making it happen.

The Houston Texans star -- who signed a $100 million contract last year and then Googled, "What do rich people buy?" -- finally figured out how to spend his cash ... beach trips with his homies.

Watt just posted a pic from some fancy beach resort and wrote, "One weekend a year, 10 best friends from high school. Always remember where you came from & who helped get you here."