I experienced the Rainforest at the Dream and loved it! My husband isn't into spas so I went by myself. I pre-booked a 1 day pass. Once onboard, you can buy a length of stay pass.
Robes, towels and sandals are provided. You get several types of exotic showers to try, sauna, dry sauna, jacuzzi, heated loungers with a view, iced water, coffee, etc. and there are different types of body scrubs for purchase (loved the chocolate one!).
You can stay for as long as you want and when you're done, there's a dressing room with regular showers and everything you need to get ready.