The four things worth packing first

If you forget almost everything else, a spa visit can still be lovely. Forget one of these four and you will feel it. Pack them first, then build the rest of your bag around them.

• Swimming costume. Bring one you are genuinely comfortable in, and bring a spare if you are staying overnight. A wet costume at four o'clock is no fun when dinner is at seven, and costumes rarely dry properly overnight after two trips through the pool. A well-fitting one-piece or tankini is worth its weight in gold in hydrotherapy pools and steam rooms, where straps and ties tend to misbehave.
• Flip-flops. Spa floors are tiled, often wet and frequently slippy. Even when the venue lends you slippers, they are usually the thin towelling sort that soak through in minutes. Flip-flops or pool sliders let you move between the pool, the changing rooms and the relaxation lounge without thinking about your feet.
• A hair tie or two. Long hair and hot tubs do not mix. Most spas ask you to tie hair back in pools and thermal suites for hygiene reasons, and there is nothing worse than realising your only bobble is in the car. Pack two — one for your wrist, one for your bag.
• A light robe. Many spas provide them, but not all, and those that do sometimes run short at busy times. A lightweight cotton or waffle dressing gown takes up very little space, dries quickly and spares you the walk from treatment room to pool in nothing but a towel.

Check what the spa provides before you zip the case

A quick look at the venue's website, or a call to reception, will save you a great deal of luggage space. What is included varies enormously between UK spas, from a small hotel pool with a steam room to a sprawling destination spa with a thermal suite across three floors.

• Towels. Often provided, but sometimes limited to one per guest.
• Slippers. Common, but sometimes only for guests having treatments.
• Robes. Usually for overnight guests, sometimes only on request.
• Lockers. Almost always available, though you may need a padlock or a pound coin for the deposit.
• Toiletries. Shampoo, conditioner and body wash are common; hairdryers, straighteners and deodorant usually are not.

If you are visiting as a day guest rather than a hotel guest, check whether the towel and robe arrangements differ. They often do.

What to wear to and from the spa

Think easy-on, easy-off. Something you can step out of without wrestling with zips in a warm changing room. A loose dress, a cotton shirt and soft trousers, or joggers and a t-shirt all work well. Bring spare underwear, plus a clean set for afterwards if you are heading straight to dinner.

Pack a decent-sized tote or gym bag for the day, and put a separate plastic bag or wet bag inside for your damp costume and towel. Your hotel room carpet will thank you.

Small extras that quietly improve the day

• A water bottle. Heat, steam and chlorine dehydrate you faster than you expect, and a refillable bottle means you actually use the water stations.
• A hairbrush and a styling product you trust. You will be glad of it after a scalp massage.
• Deodorant and dry shampoo. Helpful between a steam room and dinner.
• Something to read. Relaxation rooms are quiet by design and phones are often discouraged.
• A swim cap if you plan to swim lengths rather than float, as some pools require one.
• Light snacks. A treatment on an empty stomach is fine; one after an enormous lunch is not.

If you are staying overnight

An overnight spa break is really two trips in one: a spa day and a hotel stay. Pack accordingly. You will want something comfortable for wandering the corridors in, spare footwear that has not been near the pool, and a proper outfit for dinner if the restaurant is a step up from the bar menu. Check the dress code while you are at it.

Do not forget chargers, any medication you take regularly, and a bag for wet kit on the way home. If you have a treatment booked for the following morning, keep your swimming costume within easy reach rather than at the bottom of a tightly packed case.

What you can happily leave behind

Bulky bath towels, a full wash bag of products you will never touch, and the hairdryer — most hotel rooms and spas supply one. Leave the jewellery in the safe, as it has to come off for treatments and has a habit of going missing in changing rooms. Skip the strong perfume too: many spas ask guests to go fragrance-free out of consideration for others, and it can clash with the oils used in your treatment.

Pack light, check the details in advance, and the day itself will feel like the treat it is meant to be.