Ikeda Sensei has a video called "Za" that covers a lot of exercises for sitting (mostly related to suwariwaza). He does some pretty amazing things with his toes.
Also try pouring a hot bath and sitting seiza in the bath. This will help loosen those stingey tendons and relax the muscles.
One other thing, though this is not really an exercise. Change your diet to decrease the amount of uric acid your body must eliminate, i.e. eat less meat. Uric acid builds in the joints and causes stiffness and pain; this is one reason most yoga people are vegetarian (not that I am, just passing along the "good" news). Hope this helps.