I would recommend that you look at flavoring syrups. Monin makes a huge line of flavors, and a lot of up-scale coffee shops will add a shot of one to your coffee if you like. To make it drinkable, add carbonated water, ginger ale, or 7-up depending on the flavor.
I'm not sure exactly what flavor you are looking for, but cinnamon and ginger are probably the most "pop-py".