https://www.meeplemountain.com/articles/how-to-plan-the-ultimate-board-game-night/

Well what things can a group of adults really do that are both not lame and boring, and also not illegal

There's some non-secret societies, like Rotary Clubs, that try to do some volunteering, professional networking, and charity work together.

Also, for the under 70 crowd, there's Maker spaces.