If you lift your calves and feet off the ground and lock your knees so your legs are extending straight out and lifted off the ground under your desk, and hold them there with your thigh muscles tight, it is a good workout for your thighs.
You can even bob your legs up and down in this position a little for a better exercise.
Hi--there are many exercises to choose from that you can access quickly from an online search. They can help your specific situation--they give you quick ways to counter the negative effects of otherwise sitting at a desk and remaining stationary.
One good site is ErgoInDemand.com, which has a "Desk Exercises" page at http://www.ergoindemand.com/desk-exercises-ergonomic-breaks.htm
There also many other general sites out there with such info, such as About.com--they offer desk exercises at http://exercise.about.com/cs/exerciseworkouts/l/blofficeworkout.htm. They cover just about all your body parts--so you won't feel like a "blob"!