Caution: may result in grumpy victim.
For this prank you basically substituting the expected coffee for something your victim is not expecting, in this case, tea. Although, now that I think about, a dash of hot sauce or some dried chile peppers (in small quantities of course) might wake the victim just as successfully, if not as pleasantly, as the caffeine in coffee.
Step 1: You Need...
-the container with coffee in it
-tea to replace it with (earl grey tea is a good dark color, so there is less chance for the victim to notice the switch before drinking)
-an unsuspecting coffee drinking victim
Step 2: Substitution
If you can get loose leaf tea, so much the better, but if not just cut open the tea bags.
Place tea in the coffee container.
Step 3: Finale
If your lucky they will begin to laugh too.
If your not lucky, and they are exceedingly grumpy extend the olive branch by making a pot of coffee.