You should do pretty much your whole body if your going to do the legs. It only takes a cuple more minutes. First do your arms/shoulders. Then do side bends and half splits, both kinds. (Leg out front and legs to the sides). Then, with your bike on the stand, if you can, put your leg up on the seat and crouch down as far as you can with your one leg. Do a few of these and swich legs. Next, lift one leg and pull on it behind your back. (I find it works better if you hold your right leg with your left hand, and vise-versa) Then sit on the ground and do the sit and reach, try to touch your toes. After that put your feet togeater and push you knees down as far as you can, this really works the groin. This one is hard to explain. Put one leg over the other and rotate your back and put your elbow and arm around your knee/leg and twist, and at the same time try to pull your leg towards your body. If you have your right leg over you left, your should be tuned so your chest if facing right. Finally, if your wearing shoes, put your leg on top of he other and do circles with your ankle, then do it the other way.
Thats about it. If I know Im going for a long ride and its going to be pretty physical, I do all these before I ride.