I sit at a desk many hours a day and absolutely hate it. I like my job, just not being tied down. The funny thing is I only work 35 hours a week, and I probably only sit for about 30 of those, but if feels like 20 hours too many.
I get up every 20 to 30 minutes. In my office I'll do the plank, stretches, and roll out my tight muscles on a tennis ball.
And I also take stairs. My building has 6 flights and I'll go up and down those 1 to 3 times a day. I don't dash up them, just take two steps at a time at a nice pace. I wouldn't say it helps my fitness, but it does get my blood flow going and makes me feel better throughout the day. I'm often quite productive after a session of stretching and stair climbing.
Whatever you do just get up out of your chair as much as possible. If I have to read for a few minutes I'll often tilt the monitor up and just stand.
Oh, and if you have space, bring in an exercise ball and switch between that and your chair.