I like using an exercise ball as my computer desk chair. That helps me move around more as I'm at the computer. But there is no substitute for getting outside and taking a walk, which I think I will do just now on one of our first really pleasant, rain-free days of this year.
If you decide to get an exercise ball as a chair substitute, the advice I have received from a YMCA trainer is to size it so that your hips are higher than your knees when you sit on it. For a lot of men of near-average height, a 65cm ball will do. The size can be adjusted up or down a little by how much you inflate the ball.
I too like them. Careful though, I had been using an exercise ball for a chair for the better part of a year, just a couple weeks ago it popped while I was sitting on it.
A couple other caveats: 1)it can bit quite tiring on your back (especially at first) to sit on one for 8-10 hours. 2) since you're sitting on rubber, if it's the least bit warm, things tend to get a bit sweaty down there... :)
If you decide to get an exercise ball as a chair substitute, the advice I have received from a YMCA trainer is to size it so that your hips are higher than your knees when you sit on it. For a lot of men of near-average height, a 65cm ball will do. The size can be adjusted up or down a little by how much you inflate the ball.