The BOSU-ball is great for some leg stretches that you can do. One of them would be the hip flexor stretch. What you would want to do is bring one leg up and the other leg back, so you're in a very deep lunge, and then you put your elbows down onto the BOSU-ball, and just stretch your hip down, and you will definitely feels this, down through your hip flexors, as well as your inner thighs.
You just hold it here for a good 30 seconds, before moving on to the other leg. Then in order to stretch your hamstring one thing you can do is, you can bring your heel on top of the BOSU-ball and you're just going to bring your arms down as far as they can reach. So if you can only get to here, that's fine.
You're still going to get the stretch, but eventually you'd like to touch the BOSU-ball, if you could or even pull back on your foot with one hand so you increase the stretch. Then in order to do a calf stretch on it, what you want to do is, you want to keep your toe pointing up, heel close to the ground or on the ground, and you're just going to push down on it and you'll feel the calves stretching.