As you'll probably spend much of your time in slippers and a robe, you won't need to take a lot with you. Besides comfortable clothes to wear after your relaxing spa treatments, essential items such as toiletries and underwear should be at the very top of your packing list. Of course, if you intend to make use of the pool or steam room, or join a couple of exercise sessions, toss a swimsuit and some activewear into your bag too. Finally, leave the electronics at home and take some light reading instead. It is a relaxation break, after all.