Not a lot — but if you're staying, you may need something smart to wear for dinner, as well as your bathing suit.
Daytimes are easy; you will spend most of the day in a robe and swimming costume. Add some gym clothes and shoes for exercise sessions, plus a warm fleece and a pair of loose trousers for walking outdoors.
Evening dress varies; at some spas, people do tend to get dressed up in a smart-casual sort of way for dinner. Ditch the diamonds and low-cut dresses, though, and go for something comfortable but presentable.
At some dedicated spa retreats, it's fine to wear loose trousers and a t-shirt, but if you're staying at a hotel with a spa, the hotel restaurant(s) will have their own dress code. Check out the pictures in the spa brochure or on their website so you can get some idea, or call up and ask.
Most day spas, hotel spas, spa retreats and treatment rooms supply a robe. Beauty salons may not, but this is because they don't have wet facilities or relaxation space outside of the treatment room.
If your robe gets very damp, you can usually change it for a dry one at any time. It is good practice for a spa to provide an adequate amount of clean robes. Spa-ing is about me-time, and that is cut short if you have to get dressed and step outside straight after a treatment.
If you want to show off your family motto hand-embroidered on your own towelling robe, you can of course take it with you. But robes take up a fair amount of space in your travelling bags and may well be a bit damp when you pack to go home.
Generally, spas approach bathrobes the same way they approach towels: some big and some small. The same applies to the flip-flops or slippers spas provide. Spas need to cater to a large range of people from tall, wide men to teenage girls. If you're a lady on the large side, rest assured, spa robes are usually cut very generously, and there should be plenty of robes to fit you. Often, it's people who are smaller than average who suffer. Because of the generous cut of most robes, if you're small, you can find yourself lost in a mountain of towelling. Or trailing it down the corridor after you like a bride.
As with everything, though, if you have concerns, call the spa ahead of time and ask. A good spa will always tell you what you need to know and will go out of their way to help you enjoy your stay.
A robe is the usual dress code during the day at most spas, but what goes underneath? Underwear, a swimming costume or nothing? This depends on whether you're planning to use wet or thermal facilities, and what you feel comfortable wearing. You may well find you spend most of your day leaping in and out of the swimming pool or the sauna or Jacuzzi, so it makes sense to have your swimming costume already on. It's worth taking a spare swimsuit so you have a dry one to change into.
If you're just having a treatment, you could wear your usual underwear; your therapist will tell you if you need to remove anything so the creams and lotions reach all the right bits. For some messy treatments, such as wraps, you'll probably be offered a pair of paper knickers to put on, so you don't get your own underwear covered in muds or oils.
Many spas will give you the choice between wearing your own underwear, paper knickers, or nothing. Your therapist will tell you what's expected. The important thing to remember, though, is that you don't have to expose any bit of you that you're not comfortable exposing.
Towelling slippers or flip-flops are the order of the day. Most spas will provide disposable towelling slippers or plastic flip-flops for you to wear. Some may not, and may instead suggest that you bring your own footwear. Some spas have flip-flops available for you to purchase on arrival.
You shouldn't wear outdoor shoes near wet and thermal facilities, as guests will be walking barefoot in these areas. Dirt can easily be tranferred along the floor, and even into the pool water. You wouldn't want to get dirty feet or step into dirty water, would you?
If you're planning on doing an exercise class, you'll need to pack your usual trainers and socks; as well as whatever you wear to exercise in - a damp towelling robe just isn't going to cut it on an exercise bike.
You'll have a locker to put all your things in. If you are at a residential spa, you can opt to just leave everything in your room.
On your departure day, you will have to vacate your room in the morning but you may be able to continue to use the facilities throughout the day. Most spas will offer the use of a changing room and locker so you can leave your things there while you have one last swim.