Try adding cocoa powder, no sugar. Add the powder first, coffee liquid afterwards to minimize clumping.

The cocoa itself is quite good for you, dissolving it will cool the coffee, and it will deepen the flavor.

I should try cinnamon with this.

Edit: dutch-process cocoa powder uses sodium hydroxide to reduce acidity of cocoa. Non-"dutched" cocoa is much more healthy, but alas acidic.