An Ang Thong day trip is a strange packing problem. You are out for ten to twelve hours, you will get wet at least twice, there is a climb in the middle, and everything you bring has to survive a boat. Most people bring too much of the wrong thing and not enough of the right one. Here is the list that actually works, learned from watching hundreds of guests board.
A dry bag first, even a cheap roll-top one, because spray on the crossing and wet landings soak a normal daypack and phones do not forgive that. Then cash in baht for the national-park fee if your tour collects it separately, which is THB 300 per adult and THB 150 per child at the time of writing, plus a little more for drinks. Our entrance-fee guide lists which tours include it and which collect it.
Footwear decides more of your day than you expect. Sandals with a heel strap or water shoes handle wet ladders, beach landings and the stair climbs, while loose flip-flops slide on wet rock and end up carried. Swimwear goes on under your clothes at the hotel, because boats have little or no changing space. Add a hat that ties or grips, sunglasses with a strap if you have one, sun cream you can reapply after swimming, and a light long-sleeve layer, which does double duty against wind on the ride home and sunburn at midday.
A dry change of clothes for the return crossing is the one that separates the comfortable guests from the shivering ones, because a wet shirt in the wind of a moving boat feels cold even in the tropics. Motion-sickness tablets need to go in before the boat leaves, not after the horizon starts moving, so pack them where you can reach them at the pier. Our seasickness guide covers the rest of that subject.
Any medicine you rely on comes with you, since the boat cannot pop back. Small snacks help children and anyone whose stomach dislikes the gap between early pickup and lunch.
Passports, jewellery, laptops and anything that cannot get wet or disappear into sand have no business on a longboat bench. Bring one card or modest cash beyond the fee money rather than your whole wallet. Drones need park permission, so do not pack one assuming you can fly it. And skip the big beach umbrella and full snorkel set unless told otherwise, because tours provide the equipment their itinerary needs.
Then book the boat that fits how you travel: the full comparison shows every option side by side.