Buy a basket ball and cut a three sided square in it, depending on how heavy you want it either fill it with cloth and sand (lighter) or just sand(heavy).
Once fully filled, wrap a few lengths of duct tape around it to seal up the gap which was cut to fill.

Now, you have a great (home-made) medicine ball to train with!
Theres plenty of great exercises to do, i can't be asked to write'em up so look on a search engine or something.