Oh ya, collect all the old grinds you can find(under the grinder, on top of the espresso machine, wherever) and then pull a shot with it any taste it. Or make your co worker a "special" latte... haha awesome!
You may also ask you boss what they want done when it is slow. As a manager myself I always appreciate baristas taking the initiative and coming up wit productive things to submit to me as a part of their jobs expectations. So when you have 2 hours to kill, and you are TRULY done with what your boss expects of you, you can go above and beyond. Make a list of hyper picky things that you can do and do them then submit it to your boss. I guarantee they will appreciate your self motivation and self direction as long as you do it under their authority and for their benefit.