Hmmm, I've often put adult beverages in my coffee. Sometimes I would have a nip (50ml bottle) in a large Dunkin' Donuts coffee for the commuter train ride home. And yes, it was a long train ride allowing me to recover before getting to my car. I'd try flavored vodkas and other various sweeter liquors. Rum sounds like a possibility, though.
My favorite though is an orange liquor (Gran Marnier, Cointreau, or Triple Sec) or a raspberry (Chambord).