If you find yourself chilling in the sofa and are to way too tired for any form of physical exercise you can still squeeze in a small stretch exercise. This one is good for stretching the back of your legs and you will also get a little bonus stretch for your back.
With one leg extended, grab on to your foot with the opposite arm. You should feel the stretch on the back part of your leg as well as the opposite side of your back. How long you hold the stretch is not that important but aim for about 10 seconds per stretch and go for three repetitions on each leg.