We've never been charged anything by accepting the key to the mini bar, in fact, the CM checking us in at the Dolphin recommended accepting the key so we could put our own bottled water in there to keep cool, which is exactly what we did!
Let me see, if my memory serves....there are candy bars, M&M's, small bottles of liquor (like you'd get on an airplane) several types of canned sodas (Coke/Pepsi/Iced Tea/Sprite) several juice boxes, one was a Gatorade, one an apple juice, can't recall the other one. A few cans of beer as well. Seems like I'm leaving something out though, just can't remember!
The prices are kind of on the expensive side, but you are paying for the convenience of having it at your fingertips!