Krabi is limestone and water: climbing at Railay and Tonsai, boats out to the islands, a temple staircase that doubles as a stair machine, and mangrove kayaking inland.
Rock climbing at Tonsai and Railay
The limestone here is why a lot of people come; Basecamp Tonsai on Tonsai beach runs guided climbs, courses in leading and multi-pitch, deep-water soloing trips and gear rental, and has bungalows if you are staying to climb. Tonsai is reached by longtail from Ao Nang or on foot over the headland from Railay West.
Wat Tham Suea (Tiger Cave Temple)
A forest monastery a few kilometres north-east of Krabi Town, with a long, steep and almost entirely unshaded concrete staircase to a summit shrine over the plain. Go at first light or not at all; the monastery grounds and cave shrines at the base are free and pleasant on their own.
The four-island boat trip
Longtail or speedboat from Ao Nang and Nopparat Thara to Phra Nang, Chicken, Tup and Poda islands, with a sandbar between Tup and Chicken that surfaces at low tide. National-park entry is charged on top of the boat fare, and the longtail version is slower, cheaper and far pleasanter than the speedboat.
Emerald Pool and the Khlong Thom hot springs
Inland south-east of Krabi Town, usually done as one trip: a spring-fed pool in lowland forest and a set of warm mineral pools that step down into a stream. Around an hour's drive each way, national-park fees are charged for foreign visitors at the gate, and both are much busier after 10:00.
Sea kayaking at Ao Thalane
Mangrove channels and flooded karst inlets north of Krabi Town, paddled on half-day trips timed to the tide. Cheaper and calmer than the island boats, it works in weather that cancels the sea trips, and it is the one activity here that small children usually handle well.