Here is a simple yoga sequence that you can do to strengthen your knees:
Start in Chair pose
Hold for 5-10 breaths.
Transition into Warrior III pose.
Hold for 5-10 breaths, then repeat on the other side.
Lie down on your back and come into Bridge pose.
Come into Goddess pose. Hold for 5-10 breaths.
Come into Eagle pose
Hold for 5-10 breaths, then repeat on the other side.