Krabi is a province of limestone. The rock rises straight out of rice paddies on land and out of the Andaman Sea as islands, and that is the whole reason people come: cliffs you can climb, beaches you can only reach by boat, lagoons hidden inside islands. The province counts more than 130 islands and takes millions of visitors a year, so "undiscovered" is the wrong word. It is less built-up than Phuket, and the best beaches still need a boat, but in high season those boats are everywhere. The 2004 tsunami hit the province too, though the signs are hard to find now.
Five days is the right length because the sights here are scattered and each one is a half day or a full day: a peninsula, two island trips, a temple climb and a jungle pool. The sea decides the order. This is a beach and nature region guide, so we plan around the weather window instead of around monuments. The dry season runs from November to April, with calm water and blue skies. From May to October the southwest monsoon brings heavy rain and rough seas, tours get cancelled, and some places close for the season. If you are reading this for a trip from January, you are in the best part of the year, and also in the busiest.
The plan uses Ao Nang as the base for all five nights, with Railay as the alternative if you want the peninsula to yourself in the evenings (see where to stay). Day one is Ao Nang and its quieter beaches, day two Railay and Phra Nang, day three the Four Islands, day four Krabi Town, Tiger Cave and the Emerald Pool, and day five the Hong Islands and the mangroves of Ao Thalane. Start on a Wednesday if you can, so day four lands on a Saturday and the Krabi Town weekend night market is running. More on the country in our guide to Thailand.