Good news: you don't need a suitcase.
It's all about... you, really. The venue handles most of it — towel, locker, somewhere to shower, all the products you'd actually want once you got there. Unless you're particularly attached to your own brand of shower gel, you can leave the bathroom shelf at home.
The first time you go, you'll probably overpack out of nerves. By the third visit, you'll walk in with cash, a phone, and the clothes on your back, and that'll be the entire kit list.
What's already in there
The kit you'd actually want is in there waiting for you, and that's the whole point of bringing very little.
A towel comes with the entry fee at most places. Shower gel sits in the wet rooms. Lockers and keys are part of reception. The basics are sorted before you've even got changed.
Some venues hand out branded towels you keep, some loan you a smaller one to drop in a basket on the way out. Some places will pass you a sachet of condoms and lube on the way in. Some won't.
You'll work out which is which by the second visit, and you'll adjust without being told. The kit difference between a basic venue and a premium one is mostly the towel quality and whether the products in the wet rooms come from a bottle or a bracket-mounted dispenser. Either works.
Bring some cash
Bring some cash, every time, even when the place takes cards.
Some venues are card-everything, some are cash-only at the door, some take both for entry and go cash-only at the bar. There's no consistent rule across the country, and getting caught short at the door — having to walk to a cash point in your coat with a queue building behind you — is the small humiliation worth avoiding.
A bit more than the entry fee covers it. Drinks, snacks, the occasional surcharge for a private cabin if the night calls for it. The bar inside is where most of the cash-only-after-the-front-desk surprises happen, and being the man who can't buy a Coke is a much smaller scene than being the man who can't pay to get in.
Your own kit, if you're particular
If you've got specific gear you like, bring it. If you don't, you'll find a perfectly serviceable version of all of it on site.
The shortlist of things people sometimes bring from home: their own flip-flops, their own lube, their own condoms. None of it's required. All of it's optional.
Which one you bother with depends entirely on you. A small bag for the locker is fine if you're packing your own kit, but most men carry less than they expected to and end up walking in with whatever fits in the pockets of a hoodie.
The flip-flops, in particular, are a comfort thing — the venue's pairs are usually fine but they're communal. There's a bit more on lube and condoms on the staying safe page, but the short version: bring your own if you've got a brand you trust, use what's there if you haven't, and don't agonise over it on the way out the door.
Phone in the locker
Phone goes in the locker the moment you're past reception, and that's a hard rule everywhere.
Nobody's pulling theirs out inside the wet rooms or the lounge, and you really don't want to be the guy who tries. We get into why on the dos and don'ts page, but the short version is that phones are out, full stop, from the second your locker shuts.
Same applies to watches with cameras, smart glasses, fitness bands that record audio, and anything else that's effectively a sensor. Music goes off too — bring headphones at home if you want, bring nothing inside.
The locker key sits on a wristband or coiled lanyard you wear in the wet rooms — there's more on the locker room on the first visit page — and once everything else is locked behind it, you've already passed the hardest part of getting changed.
What you can't pack
A bit of patience with yourself is the thing nobody actually packs and the thing every first-timer needs most.
The first half hour is always the weirdest. You'll be over-aware of the room, of the locker key on your wrist, of the towel you can't quite work out how to tie. After thirty minutes, you'll forget you were ever nervous about any of it.
That's the part that doesn't fit in a bag. It's also the only thing you'll wish you'd brought more of, and the only thing the venue can't provide for you.
When you leave
You leave with what you came in with: wallet, phone, the same clothes, slightly damper hair.
The venue keeps its own kit. You walk out with yours.
The next time, you'll bring less.