I grew up playing every sport you could possibly imagine. So, as an adult who misses being on teams and the competitiveness of sports, I looked into joining adult rec leagues when I moved to San Francisco almost 4 years ago (jeez, how has it been that long?!)
I've played volleyball, soccer, softball, kickball, etc. in a variety of different leagues and have really enjoyed them. Most of the leagues take place 1 night a week for the duration of the season (usually ~7 weeks + playoffs) and typically also have a social aspect to them. You'll often get discounts at a specific bar to go to and socialize with the other teams after the games and there are sometimes league wide events like wine tasting, game nights, Giants game outings, etc.
If you're new to the city, it's a great way to meet people too! You can join the team as a group if you have a big group of friends who want to play, as a single and tack on to another team, or a combination of both (if you come with only 6, the league will assign other people who signed up alone to make up the rest of your team). They typically run anywhere from $60-100/person, depending on how many are on your team and whether you take advantage of the 'early bird' specials.
You don't need to be a professional athlete to play in these leagues - there are various league levels from beginner to advanced, so if you're just looking to join as a way to meet people or get together with your friends and simply have fun, there's definitely a league level for you!
Here are my favorite leagues in the city:
basketball, cornhole, dodgeball, kickball, soccer, softball, and volleyball
basketball, bowling, dodgeball, football, kickball, softball, soccer, tennis, and volleyball
Co-ed sports league where you keep the same team but rotate and play a different sport each week throughout the duration of the season.
This is a really competitive volleyball league, so if you played club growing up and want to some solid competition in vs. just the fun aspect, check this one out! My personal favorite. :)
PS: If you need gear, you should check out Play it Again Sports. You can get gently used equipment for really cheap! They have a few Bay Area locations (unfortunately, not in the city, but worth the drive!)
Go get your game on!