We've become really close friends with a couple of families on our block. It definitely helps that we all have kids the same age, but a big reason for this is that our kids entertain each other, freeing the adults (most of the time) to interact with each other.

I also sing in a small choir that is as much a friend group as it is a music ensemble. Most of us have kids, but not everyone; while I can't speak for those in the group without kids, I think one reason it works so well is because none of us have parenting responsibilities while we're together as a choir (we joke that rehearsals are a great time for us to escape from our families).