Given you'll probably spend much of your time at the hotel in a fluffy robe and slippers, you won't need to take a lot with you. Besides casual, loose-fitting clothes to slip on after your treatments, essentials like underwear and toiletries should be at the very top of your packing list. Of course, if you intend to make use of the hot tub or pool, or engage in exercise sessions, toss a swimsuit and some gym attire in your bag as well. Finally, leave your electronic devices behind and bring some light reading instead. It is, after all, an escape from the real world.