You mentioned it, but I'll go deeper: the dog park.

In reality, this applies to any third place, but we'll go with the dog park.

I've been going to my local dog park nearly every day for the almost 14 years. Over time, I've met many of the best friends and best humans that I've known in my entire life. I have a group of regulars that evolves over time, and we're all good friends now. I also regularly see acquaintances who show up less often. This has been fantastic for my mental health.

So, again, this really applies to any third place. Go to church, a lot, if that's your thing. Join a bowling club and get deeply involved. Join a climbing gym and go all the time, and strike up conversations with people. Join a martial arts organization and get seriously involved.

The keys are to socialize and to fill up your time.