There are several exercises you can do while sitting down. I sit at the computer desk a lot, so I've purchase a foot peddler. You can use it while you are sitting down in a chair, whether you're watching television or on your computer trying to get some work done. It's a great way to tone your legs while you're sitting, especially if you do not have a lot of extra time to exercise.
You can also do a few different movements while sitting on a chair. Sit down, spread your legs as far apart as possible, and bend down towards the floor while remaining seated. Hold the position for several seconds, roll your body back up and then repeat the steps. You can even do leg lifts and modified squats while seated on a chair, so there are options!