Not far from Phuket, just south along the coast, is Krabi. There's a cluster of tourist places around here, Ao Nang is a popular tourist beach, and Koh Hong is a short boat ride away, with it's large lagoon.
The big tourist things to here, Tiger Temple, Wat Tham Suea - the attraction isn't the temple, it's the hill with 1237 steps up to a pagoda and Buddha statue, and some spectacular views. This temple is an absolute must do, but its hard work. The beaches, are a draw and Phangna bay boat trips, there's a hot spring, hydrothermal pools you can sit in and take the salts, and the Emerald Pool, a green mineral natural pond for swimming in, with a blue pool and nature area nearby for viewing.