A stretch that's an oldie (but a goodie) is the standing quadriceps stretch. You definitely did it in middle school and you'll always see runners doing it after a run, but here is a refresher to make sure you're getting the most out of your stretch.
Here's how you do it: Stand upright on one foot. Hold the other foot with the opposite hand (you can use the same side hand or both, but using the opposite is better for your knees) and raise the heel of the lifted foot to the bum. If you can't touch the booty, get as close as you can comfortably without causing pain. Keep your body upright throughout. Hold for 20- 30 seconds. Now, switch sides and repeat.
Fit's Tip: Use a wall or stationary object for balance if you can't balance on your own.
My son uses ME to balance when he's doing this since he can't do it on his own yet. I keep having to remind him that I don't mind, just not while I'm in the middle of said stretch!