Millennium-class ships are the largest, newest, and most feature-filled in the Celebrity fleet. Innovations include the Conservatory, a unique botanical environment; show lounges reminiscent of splendid opera houses; and alternative restaurants where diners find themselves in the midst of authentic ocean liner decor and memorabilia. The spas simply have to be seen to be believed -- they occupy nearly as much space inside as is devoted to the adjacent outdoor Lido deck pool area. Although the spas offer just about any treatment you can think of -- and some you probably haven't -- they also house a complimentary hydrotherapy pool and spa café. These ships also have the most to offer for families, with the largest children's facilities in the Celebrity fleet.