PSMKay's original post was regarding a work environment that's predominantly sedentary and how to avoid that, so I don't think we're veering too far off topic by branching slightly into personal life (After all, we only have one body. Our physical health is our health whether we're in rehearsal or at home). A lot of the conventional wisdom over the last couple of years suggests that we should shoot for 10,000 steps/day to avoid a sedentary lifestyle. There are a number of relatively affordable devices out there that track our steps (e.g. Fitbit or Jawbone), I found wearing a pedometer a good way to measure how active I've been, and perhaps more importantly how inactive I am some days. Some devices will even buzz you if you've been sedentary for x number of minutes.
Once you start to get into the mindset of reaching 10,000 steps/day you find ways to make it happen, even if your job is mostly sitting. Take the stairs instead of the elevator, park at the back of the lot, get off one transit stop before yours and walk, etc... At the end of the day if I'm short of 10,000 I will walk around my living room while I watch TV instead of just sitting on the couch. I'm sure it looks pretty goofy to my neighbors in the next building over, but it's a good way to get in some extra steps while relaxing.