"Trang An is a boat trip rather than a walk. A small rowing boat takes four or five passengers through a chain of caves cut under the limestone, stopping at temples on the islands between them. The whole circuit takes two to three hours and you stay in the boat for most of it.
There are several routes and they differ. One takes in the film set built for Kong: Skull Island, another visits more temples, a third passes through the longest chain of caves. You choose at the pier, and the choice matters more than the price, which is the same.
People often confuse Trang An with Tam Coc a few kilometres away. Tam Coc is shorter, passes through three caves and runs between rice fields that turn gold in late May and June. Trang An is longer, has more caves and more temples, and the water is enclosed by cliffs rather than fields. If you have one morning, most visitors prefer Trang An; if you are here in harvest season, Tam Coc is the one worth adding.
Two practical things. The caves are low in places and you will need to duck, sometimes for a minute at a time. And the rowers work on a rota for a small fixed fee, so a tip at the end is customary and genuinely welcome."