Many of us will spend all day sitting at the computer. If we don't get up and exercise now and then, all that sitting will lead to fatigue, weight gain and worse. Here are some ideas for office exercises - exercises you can do right at your desk or workstation. My personal goal is to do at least one of these ten minutes exercises at least every two hours. It makes a world of difference in my energy level and makes me more productive when I get back to work.
Push ups are a great strengthening exercise. If you can find a clean and big enough spot on the floor, by all means do some push-ups every day. If you can't (or don't want to), try some countertop push-ups instead. All you do is just put your hands on a desk or countertop (one that won't move) and do as many push-ups from that position as you can. In a pinch, you can use a wall. The idea is to max out the number of push-ups at least once. Repeat this often throughout the day. I find a nice energy boost from doing just 15 or so of these.
If you're able to keep a fitness ball near your workout station (I work at home, so this is no problem), then these workouts are great. You can do many of them in your "work clothes." These exercises will strengthen your back, abdomen and core in no time. If you don't have a fitness ball handy, try to adapt these while sitting on the edge of your (unwheeled) chair. Some twists and crunches can easily be done from any chair.
My favorite office workout is using these suspension training straps. Basically, these are just straps that you put behind a door. You can then do a full upper-body workout by leaning into the straps and lifting against your own bodyweight. The straps take up no space, but you need to be careful that no one opens the door during your workout. If you have an office with a door you can lock, you're all set. The best part is that doing just one set generally doens't make me sweat enough to need to change clothes and if I repeat throughout the day, I end up with a good uppoer body workout.
Bodyweight exercise is a no equipment exercise that uses just the weight of your body for a full workout. Some of these exercises would require a change of clothes, as bodyweight exercises can be very strenuous. However, these exercises can easily be adapted to a small space, especially since no equipment is needed.