Sadly, the Rainforest Room is not free on boarding day. I think you maybe can walk around the area during the spa open house time? You can purchase a day pass onboard or you can purchase them in advance through the website. You have to order them through the same part of the website where you purchase gift baskets or water bottles for your room, not the excursions area.
Personally, I loved the Rainforest Room. On our 4-night cruise on the Disney Dream, we skipped the sail away party and watched the ship sail away from the hot tubs overlooking the ocean since we had seen the show before (and were cruising without kids). It's all a matter of preference. There are 3 saunas and 3 (or 4?) shower options, with scents and different water pressure/temperatures. In my opinion, the best parts are the hot tubs that look out onto the ocean and the heated tile loungers. I think they're super relaxing and could spend all afternoon in one.
Hope this helps!