I like running, so I do that. We have some great overnights for running. If the hotel has a good gym I'll go lift weights and relax in the pool. After that I like to find public parks nearby to go wander or I'll walk around town and see what I can find. Historical stuff, interesting buildings, people watching, etc. Some of our layovers have really great hikes nearby. I always get takeout for dinner and it's usually somewhat far away, so a lot of my time is spent walking to and from the restaurant. I try to avoid spending money on activities as my budget right now doesn't support that.
I'm a social guy but I'm kind of a slam clicker on overnights. I use the time to decompress. I did the bar scene in my 20s and got pretty burnt out by it. But I will jump on an opportunity to go out with the crew if they're doing something other than dinner or drinks.