Firstly, check if your spa hotel has any sort of dress code. It's likely you'll be given a robe and slippers, but not necessarily. Underneath, you should aim to be as comfy as possible; track pants, yoga bottoms, sweatshirts and T-shirts are often encouraged. Trainers and sports shoes are usually permitted, but limited to dry areas. It's also smart to bring another swimsuit or some extra clothing along with you.