One common problem with cycling is knee cap tracking. I occasionally get a sore knee. When I do, there is one exercise I like. I sit on the edge of my bed and hang my feet over the side. Then I slowly raise my foot until my lower leg is parallel to the ground. That's it - easy and no weights required. What you're doing is working the support muscles that keep the kneecap tracking properly. Sometimes it may help to use your hand to make sure the kneecap it moving the way it should, but for the most part it's just an exercise in holding your knee squarely as you raise and lower your feet. If you do this a few times on days preceeding rides, it might help surprisingly.