When you need to stretch the legs, shoulders, core, and legs, try Three Legged Dog. This pose is done from Downward Facing Dog and is a great way to isolate the legs while building upper body strength and getting a good stretch in a variety of places. Many yoga teachers will use this pose in the beginning of the class and as a transitional pose between postures after Chaturangas.
Starting in Downward Facing Dog, lift the left leg parallel to the floor. This is the first phase of this pose. You can hold this pose and benefit the standing leg by trying to press the heel toward the floor. This pose will provide a phenomenal stretch in the standing leg. You will also work the core muscles and the thigh muscles to keep the lifted leg up in the air. Then, repeat on the right side.
You can take the pose to several other places. Lift the leg as high as possible, while keep the integrity of the Downward Facing Dog pose. You can add a bit of a twist in the oblique muscles, but try to keep the arms square. You can then, bend the lifted knee and build rotation in the core and shoulders, while keeping the hands on the floor. If you really want to have some fun in the pose, drop the bent leg all the way to the floor into a strong backbend called Wild Thing.
Some people feel skittish lifting the leg to the sky. If you find yourself being nervous about the pose, just lift the leg to the hip height, so you can build strength. As with every yoga pose, there is no reason to hurry into a deeper version. Yoga is a life-long practice, so you have plenty of time to develop your poses.