This brings up an interesting series of thoughts. What goes on in your mind is both the same and different from what I think about. One of the reasons I am wireless is the ability to wander around during soundcheck. It's not unusual for me to be responsible for the sound, since I'm the fool who owns a PA. I do a lot of jumping around on stage, even with the upright, when it's appropriate. With some bands, that would be distracting, so I don't do it. Given a choice, I'll dance around because it is part of playing and keeping time. For me anyway. There's a lot of on-stage communication that goes on, most of it serious, but some of it very playful. In one band, the singer and I have a standing joke that zings back and forth sometimes. It doesn't distract from playing, but it keeps us from being stiff. She has some stage fright, even after performing for decades, and that kind of thing helps with it. I also scan the audience. Making connections with people in the audience is fun. So is having energy bounce back and forth between the stage and audience makes it all worthwhile.
So, lots of things go on in my mind. There's also the OH, CRAP! I forgot the next song - we have only played it six million times and now I forgot! ... moment. Muscle memory kicks in often enough that it's usually not a problem, but the occasional train wreck makes for a lot of *** looks around the stage.