Swiss ball exercises - you can do just about any weights exercise on one.
Planks - God's gift to the sadist personal trainer.
Bosu ball exercises - the Bosu ball looks like half a swiss ball with a plastic surface. Just balance on either surface (squashy side down is easier). When you get the hang of it, get a medicine ball or a light weight and move with it - up and down, side-to-side, over the shoulder and down to the floor.
If you can talk to a professional and/or have access to a gym with the equipment, so much the better