First and foremost, find out if your spa hotel requires you to stick to a dress code. It's likely you'll be supplied with slippers and a robe, but not always. Underneath, aim to be as comfy as possible; track pants, yoga bottoms, sweatshirts and T-shirts are often recommended attire. Trainers and sports shoes are normally allowed, but limited to dry areas. It's also smart to bring another swimsuit or some extra clothing along.