As someone who visits the Rainforest Room at least once on every cruise and has been known to purchase length-of-stay passes, here's a quick run-down as to your options. There are typically three different types of passes you can purchase, listed as follows:
1) Advanced Purchase ONLY Day Pass for $16. This is good for one day - you can check in and out as many times as you want. They will put a sticker with the date on your room card so that they know you've checked in. You can purchase as many of these as you want ahead of time. They can be used by anyone in your cabin. You can also give them to people in other cabins, simply by visiting the spa and letting them know that's what you want to do.
These passes do not come with any scrubs. However, on our cruise in May, we were asked if we wanted to add the scrub for I think it was $9.00 (per visit). I chose to pass.
2) Length of Stay Pass: These passes are good for the entire cruise, and can be a good value if you're planning to visit the spa a LOT during your cruise. It's also kind of neat that the spa staff gets to know you - kind of an 'in crowd' feeling. (Though I've gotten this same feeling when I used a number of day passes on a single cruise). They only sell a very limited number of length of stay passes, so if you know you want one, buy it on the first day to a) get the best value and b) insure that they don't sell out. (On our WBTA this spring, they did offer slightly discounted length-of-stay passes a few days into the cruise).
Sometimes these seem to include the scrubs, sometimes they don't. The best thing I've found is simply to ask up front before I buy the pass. I have purchased both solo and couple length-of-stay passes, and they have never included a Rasul spa treatment (or any other treatment, besides the scrubs you can use in the Rainforest Room)
3) Day-of, Single Day Passes. These are single day passes purchased while you're on the ship. These May or May Not be available on the specific day you want to buy them. From what I understand, whether they are available depends upon how many people are currently in the Spa. I think the idea is to make sure it's not too busy for the people who have paid the $ for length of stay passes, but to allow additional access when it's more empty.
When you CAN purchase a day-of/single-day pass on board, they typically cost $25 and automatically include the scrub. So, prebooking is nice because it's less expensive if you don't care about the scrub, and you know you'll be able to go. (I've never been turned away when I go to use my prepurchased day passes.
On a related note, when you prepurchase Spa Day Passes, you will receive a card in your room stating how many passes you've purchased. You can bring that card with you the first time you visit the spa. But it's not truly necessary because they have a sheet that lists out all of the prepurchased passes, and will check you off on that list.
Hope that helps!