I like to do medicine ball slams.
I do them a bit differently, our balls bounce so I catch it as it bounces up.
Abs--sit down facing each other, one person rolls to her back, sits up and throws the ball to the other. Repeat. Try throwing at different angles to each other to get the obliques.
Stand a distance from each other. One person rotates and throws the ball to the other.
my favorite partner medicine ball exercise is each stand on bosus and throw the ball to each other trying to get the other to step off, i.e. throw it almost out of their reach, throw it hard, fake them out...
no fair to just throw it totally out of their reach.
How about if we do them on balance disks?