I think there's something in the air today, b/c everyone (online) has been mentioning how slow it feels. I haven't worked in an office environment since June of last yr, but I'll tell ya what I used to do & give ya some advice.
1. We had scheduled breaks, so I'd always go for a walk on my break. When I had like-minded friends, we used to do sprints in the (infrequently used) stairwells ;)
2. Fidgeting...lots of it! I'm probably the worst knee-bobber, toe-tapper, finger-drummer out there...I fidget constantly, which I figure burns an extra hundred calories a day (if not more).
3. Never use an elevator if you don't NEED one, and always take the longest route anywhere. I would always park at the faaaaaaaaaaar end of our parking lot to maximize my walk into the building. And luckily, our building only had 3 floors, so I never had to take the elevator unless I was hauling tons of paperwork somewhere.
4. Be more social. If your job allows it, get up and go talk to someone instead of emailing them. Company email makes things convenient, but sometimes it's good to talk it thru, too.
That's all I can think of at the moment. Don't worry about sitting on your duff all day. So long as you are getting your exercise in, you'll be fine.

(Try to) have a great day!