The chocolate cake recipe handed down to me is very similar to the Hershey's cake, except 1) it calls for a cup of buttermilk or sour cream instead of milk; 2) it calls for a cup of boiling water with a heaping teaspoon of instant espresso powder mixed in. (Actually, it calls for instant coffee, but I use espresso for a stronger flavor.) Come to think of it, my recipe might be an altered Hershey's recipe!
To spice things up, I've added Kahlua, Amaretto, Liquid CoffeeMate creamers (hazelnut or raspberry are amazing!), Orange Liquor (Grand Marnier, Triple Sec), Chambord, and on and on. I add these to the buttermilk/sour cream or boiling coffee, making sure the combination measures 1 cup total. Also, keep in mind that buttermilk has a stronger flavor than sour cream, so some "add ins" don't go as well with it. To be safe, just use sour cream.
Edited to add: If I'm adding a clear liquor, I usually add that to the boiling coffee; if adding CoffeeMate, Kahlua, Bailey's, etc, I usually add that to the dairy.