Five-a-side football (soccer). I'm not a sporty person, but started going with a local group a few years ago and have reaped the benefits of doing some intensive team exercise once per week. I go with a bunch of guys way older than I am, and it's amazing how fit and healthy they are compared to the average person I meet of their age. I certainly plan to keep this up so long an injury doesn't prevent me.
Five-a-side is a specific format of football (soccer), aimed at more casual play with a much lower bar to skill level. Outside of five-a-side leagues (which do exist), it's rarely played with fixed teams and often ran in a more "pick up group" fashion.