Emerald Princess has great free sauna and steam and in my experience has a good amount of traffic.
HAL Koningsdam, Niew Statendam and Rotterdam as well as Cunard Queen Elizaberh and Victoria have a great free sauna in mens locker room with large qindow onto the ocean. Have found these ships all have a good social crowd especially mid to late afternoon.
All Jewel class ships on NCL have a great mens only fee paying spa area which I feel is well worth the money, they also have a free steam room in mens gym locker room.