The Roman Baths are one of the best-preserved Roman sites in northern Europe, and the Great Bath, green, steaming, surrounded by Victorian-era statues of Roman governors, is extraordinary in a way that photographs only partially convey. The complex includes the spring itself, the bathing rooms, the hypocaust system, and a genuinely excellent museum with objects dredged from the sacred spring over the last century.
The audio guide narrated by Bill Bryson is excellent and included in the price. Come at opening time (9am) for the lowest crowds. Evening events with torchlit openings run throughout the year and are worth checking.