How do non-gym-goers stay fit?

I feel like every adult goes to the gym. If you don't, how do you stay in shape? Ideally I'm looking for something aside from running. My knees aren't great anymore.

Edit: I'm just asking because I'm curious what else people do. I do simple cardio and strength train at the gym when I do go but I was curious about the habits other people maintain outside of the gym aside from things like running. Sometimes due to my mental health I don't go to the gym for weeks so I just wanted to see what else people are doing. I didn't expect this many responses. Thanks everyone for sharing.