Most of this comes down to one rule about shoes and one thing nobody tells you about harnesses. Here is the whole list, which is shorter than you would think, because the parks supply everything that actually matters.
Every park requires closed-toe shoes and none of them will bend on it. Trainers or walking shoes are what you want. Sandals, flip-flops, slides and Crocs will get you turned away at the registration desk, which happens more than you would imagine to people who have come straight from a beach hotel and assumed a zipline was a sitting-down activity.
The thinking behind the rule is easy enough to follow. You spend a fair amount of the tour walking between platforms on wet wood or dirt paths, and on several courses you are climbing nets and staircases as well, so a foot that can slide out of its shoe at height becomes a genuine problem, quite apart from what happens to a loose shoe that drops off a platform.
Jungle Xtrem is the useful exception here, because it rents sports shoes free in sizes 32 to 47, so anyone travelling light or who has already made this mistake has somewhere to go.
The whole specification is something you can move, climb and sweat in, and shorts or leggings both do the job. Length is the only thing worth thinking about, because the harness leg straps sit high on your thighs and carry your weight for a couple of hours, and against bare skin under very short shorts they rub. Longer shorts, leggings or light trousers are considerably more comfortable, and it is the detail people most often say afterwards that they wish somebody had mentioned.
Trousers have a second advantage in the Kathu valley and the Chalong plantation, where the mosquitoes in the late afternoon are not a rumour.
On top, a t-shirt is fine, though avoid anything loose or flowing since it catches on the harness and the hardware. Pack a spare shirt for the drive home too, because you will finish damp whatever the season is doing.
Nothing needs buying before you travel. Helmet, harness and gloves are supplied everywhere and fitted by the guides, and Jungle Xtrem throws in shoes on top of that, so the pre-trip shopping list is genuinely empty.
Anything that can fall off you. That means sunglasses without a strap, hats without a chin cord, loose phones and jewellery, because things dropped from a platform forty metres up are not retrieved from the canopy afterwards.
For photographs you have a few sensible options. A phone stays safe in a zipped pocket you simply do not open while you are up there, a camera works on a proper wrist strap, and most parks sell their own photos taken from angles you could not reach anyway. Several also rent action cameras with a harness mount, which is the only genuinely secure way to film yourself flying.
Tarzan provides lockers and most other parks have somewhere secure for a bag.
Put both on before you leave rather than in the car park. Sunscreen matters most at the exposed parks, since Erawan sits on an open hillside above Patong and the sea-view platforms on the Rawai course have no canopy over them at all, while repellent earns its place at the forested ones, Flying Hanuman in the Kathu valley above all.
Order matters more than people expect. Sunscreen first, let it dry, repellent second, because doing it the other way round lifts the repellent straight off again.
Both are fine. The wind on the faster lines will make your eyes water, so bring drops if your contacts dry out easily, and anyone in glasses wants a strap, since a helmet strap and a zipline together are not something an unsecured pair of glasses tends to survive.
Between May and October the thing to add is grip. Wooden platforms and dirt paths turn slippery after a burst of rain, and a pair of trainers worn smooth underneath makes the difference between walking the course confidently and picking your way round it.
A rain jacket is not worth packing. Light rain does not stop the ziplines and the parks carry on running through it, and you will be dry again twenty minutes after you finish anyway. Heavier rain means waiting under cover for a while, and lightning stops everything regardless of what anyone is wearing.
Closed shoes, longer shorts or leggings, sunscreen and repellent on before you arrive, nothing loose in your pockets, and a spare shirt in the car. Everything else is provided.