The Rainforest includes the loungers, the steam rooms, the sauna, and (on Fantasy and Dream, the oceanview hot tubs. The tile loungers on the Fantasy and Dream have a view of the ocean as well.
I think a seven day pass on the Fantasy runs around $80 to $90, and a couples pass around $150. A day pass, if available, is around $16.
To book a day pass before travel, go to "In Room Gifts and Shopping." The pass is considered a thing to purchase, not an activity, so you will not find it under excursions and dining reservations.