A good strengthening exercise for both your legs and your core is lunging from a plank position. You would start in a plank position on your hands or on your elbows, depending on which is most comfortable for you and then you just bring one leg into a lunge position. It's a deep lunge, so you want to make sure your knee is not past your ankle, it's right in line.
This actually will open your hip flexor at the same time, so it's kind of a good stretch, it might feel really nice. Then you bring it back. You do the other side. You just bring it all the way forward and all the way back. Then in order to make this more cardio, you can jump lunge. So you would bring it in, jump to the other side and jump to the other side. That would be a lunge from a plank position on a BOSU-ball.