The Secret Lagoon, on Miniloc Island, is the smallest and most theatrical of the El Nido lagoons: a circular pool of shallow water completely enclosed by vertical limestone walls, reached through a knee-high crack in the rock that you have to wade or swim through. Inside the lagoon there is barely room to stand; the cliffs rise nearly vertically all around you.
It is included in standard Tour A and gets packed at peak times. Aim to be one of the first boats of the morning if you want it to yourself for even a minute.