This is a very pleasant Krabi city trip. From the pier next to the Black Crab (Boo Dum) you take a longtail boat upriver between the two distinctive limestone peaks. The boat will drop you (and wait for you) at another pier from which there is a short walk (sometimes with monkeys) to the staircase which leads up to a large cave. Some dinosaur bones have been placed inside, as well as statues of cave dwellers (despite the millions of years difference in age!) but the cave is large and has a number of paths, some enabling you to return down the same staircase, and some leading to an alternative ladder way down.
Once back at the boat, the next part depends on the state of the tide. If high tide you should be taken back past the Black Crab, but on the other side of the river, and then into a smaller branch. From there you will be taken into a very small creek, only passable at high tide, through the mangroves where, again, you may see monkeys. At the end of the creek you reach a major waterway again, on the opposite side of which are several fine seafood restaurants from which you can choose. The boat will wait for you, as he gets a small commission, and return you to the Black Crab pier. Cost should be Bt 500 per boat (not per person).