For starters, check if your spa hotel has any sort of dress code. You'll most likely be provided with a bathrobe and slippers, but this isn't always so. Underneath, you should aim to be as comfy as possible; yoga bottoms, sports pants, loose-fitting sweatshirts and T-shirts are often encouraged. Trainers and sports shoes are usually allowed, but limited to dry areas. It's also smart to bring another swimsuit or an extra set of clothes with you.