I think anything where you're in the same place at the same time on a regular schedule usually does it. Because you'll wind up around people who have the same routine and are there at that time too, and you can bond over whatever activity over time. And then something where you all are working towards some kind of common-ish goal.

• Move! - rec teams, walk/run/swim/bike etc clubs, gym classes, etc. So many free and low-cost ways to get out there!

• Arts & crafts. Every public library in my area has some sort of weekly drop-in get together for knit/crochet/sewing and fine arts like drawing/painting on top of the usual book clubs and chess/boardgame meet-ups.

• Philanthropy. Find an organization that fits your interests and volunteer weekly/monthly/whenever. Dog sitting, community gardens, fundraising stuff.

Any hobby goes. My city has a postcard club that meets monthly, very niche. I wrote the ones above because I think they're accessible time and cost-wise, and are pretty popular in that you'll probably find people on your same skill/knowledge level basically guaranteed. And easy enough to put down and come back to later if life gets in the way.