One thing I always make sure to do before lunch every day (it isn't raining) is get outside for a walk. I consider this the most important thing I do at the office each day.
The obvious reason to walk is that it's a form of exercise, and that has all sorts of health benefits. It certainly is better than sitting at a desk all day. One of the reasons I started doing it was seeing the occupational consequences for others, and realised I was heading down the same path.
It's not always easy to make time to exercise, so my lunch break serves as a time I can dedicate to it without needing to juggle other things around. If it becomes habitual, one never needs to make time for it.
And as far as exercise goes, walking is not that strenuous. One can walk (and even walk fast) without breaking a sweat, which makes it a convenient form of exercise for the office.
There are benefits besides the fact that it's exercise, and exercise is good for your health. Walking has been found to help with learning - that it can increase memory retention. For my personal experience, I've found it's a great way to collect my thoughts, and even solve problems. Many times I've been stuck on a problem, only to solve it 5 minutes into my lunchtime walk.
If you are walking outside, there's the added bonus of getting vitamin D. Not getting enough vitamin D is an unexpected occupational hazard of working in an office, as incidental exposure to the sun (our primary source of vitamin D) is much less than in other professions. There's even research suggesting that walking through areas with trees has benefits over walking through streets.
Finally, walking provides an excuse to get away from the office, away from coworkers, and away from the work itself. It gives me a break, and a chance to take my mind off work-related matters. And best of all, it's undisturbed.