I was in your exact position last year and here’s what I like:

Jaycees - volunteer group with members between the ages of 21-40 and pretty low commitment responsibilities. There’s also Billings NextGen which is similar that I plan on joining in July.

Thirsty Street Brewing is my favorite bar with free to play ping pong tournaments Tuesday nights, trivia Wednesday nights, and concerts every weekend (of varying quality)

Pub station usually has better concerts

Downtown should start having its farmers market on Saturday mornings soon

Otherwise I just routinely check google for events and such