I have been going to this outdoor spa for years, and it just keeps getting better. I am not terribly familiar with the rooms, although they are probably nice enough, but I usually just go for day use since we live not too far away. It is like a little country club with mineral pools and spa services, and a gym. Get there early on popular days, and enjoy the tubs and pools until 9pm. They just renovated (completed December 2011), and it looks better than ever - all the tiling looks beautiful and tastefully done. They added a gorgeous fire pit, beautiful indirect lighting, and it looks like they are planning to add some outdoor canopied beds if I am not mistaken. Bring a picnic, or walk one block to get to downtown Calistoga and buy a picnic lunch and bring it back (there is even a winetasting place within a block). They allow you to bring food and drinks (alcoholic too!) to enjoy at the tables that surround the pools. This place is not only great, but kid-friendly. There is a large gazeboed hot tub which is adults only, but they have a kids warm pool under a waterfall, and the lanes pool and other hot pool are open to kids (they even allow flotation devices).
Oh, did I mention that on January 1st when we went, it was 70 degrees out and sunny? Little slice of heaven.
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