The dining plan typically refers to the standard non-alcoholic fountain drinks such as soda, coffee, tea, iced tea, lemonade - basically the drinks that come with free refills at TS or the ones that you find at food courts and QS that are filled from a dispenser. The Disney owned restaurants often have additional choices so if you see something on a menu that interests you, ask if it is included. We've had milkshakes, smoothies, specialty coffees and teas, pressed pot coffee and non-alcoholic cocktails at various restaurants.
The non-Disney owned places have fewer options because they are reimbursed according to the rules of the plan so there are not many options beyond the standard fountain beverages.