The rainforest room is $15 for a single day pass or $75 for unlimited use during a whole week. DH and I each bought an unlimited use pass and we were in the Rainforest every day - we loved it!
You just check-in at the desk and tell them what you're there for - they will take your key to the world card and give you a basket with a robe and a locker key. You can change into your swimsuit and wear the robe over it into the rainforest room, or if you're like DH and me - just wear your suit into the spa and throw the robe on over it (we always went after spending some time at the pool ).
The main room has four tile heated loungers, a fountain, and a heated tile bench along one wall. The loungers are pure heaven! There is also a steam room, sauna, and a gentle (or dry) sauna. We did not care for the steam room or regular sauna, but the gentle sauna was fabulous! It's a separate, small tiled room with heated benches and heated footrests - and of course the room is very warm (deliciously warm, but I'm always cold). There is also a small fountain with a hose that you can use to cool yourself with.
I know there are some showers in the rainforest room, but we didn't use those. All I know is that I wish I had a rainforest room, especially the gentle sauna, in my home.
We did the spa taster as part of our REAS package and the massage part was very nice - we've had couples massages before, on land, and those were definitely better than the ones on the ship. Personally, we enjoyed the Rasul treatment more than the spa taster. If you're going to chose one over the other, I'd vote for the Rasul. It's more affordable and the two of you get a treatment suite to yourselves with no attendant for an entire hour.
Just my two cents!
B.