My friends and I spent most of our lives since grade 9 doing school stuff or stuff to get into school stuff. In my case, couple that with introversion and an aversion to most normal activities and the easiest thing to do is be good at the get into school stuff.
In university, most of my social activities revolved around the startup world, hackathons, or other competition-oriented activities as that is what I was good at. I went to one hackathon a weekend in my final academic year.
So I became good at a lot of these types of things which all revolved around the academic world.
I am not in university anymore, but I can still fill my time with the kinds of competitions and challenges in that world. If I couldn't, I doubt that I would know how to use my time. Even with those, I spend a ton of time commenting online.
Most of the people I know are in the same kind of boat. They were good at one or two things (might be science fair, research, conference organizing, etc) back in university which consumed their time. Then they left and are now left wondering, "now what?"
Like taking up some challenges in life. Besides maintaining physical fitness and getting tired in the process, one thing I like is to create contraptions (does not have to be a "product") that could be useful to you or people you know. An air purifier, a charging station for a phone, a piece of wooden furniture, or anything else. Get some tools and get working.
If you prefer social activity, plan to do the above with a friend.
Then you ssh in and work to fill the hole. Now I don’t even get to see my co-workers.