Karate. I did some martial arts when I was a kid, so when my son started with karate I was of course supportive. He has some minor special needs, so he preferred if I was present during his sessions, and this ended up with me getting to know the group pretty well, and I ended up joining myself.

I have since dropped out, mostly due to moving far away from a dojo practicing the same style, but there and then it was pretty fun to join in on the anual tournaments despite not being nearly as flexible as I was in my teens.