Our student recreation center has started purchasing some of the newer, rubberized medicine balls. I've added them to my pre-class workout, and I have really enjoyed the variety of exercises I can do with them. They are great for abdominal workouts and you can add them to lunges to give an extra challenge. Throwing the medicine ball with a partner works great as well. At the very least, there's a few exercises that you can add to spice up a routine when you get bored.
I wanted to link to a few exercises I found on YouTube that you may enjoy as well.
The lunges look pretty easy at first, but the fatigue starts to settle in, and the shoulders get a nice burn as well. I really enjoy the medicine ball push-ups since they force me to move my weight around while keeping my core tight.
Just don't try THIS exercise:
Though, perhaps this routine might have some overlooked martial arts benefits after all! :)