South of Krabi you have the laidback Railey Beach. Although Railey is not an island, you can only access Railey by long tail boat, from Ao Nang. Many Railey's visitors are backpackers from all over the world, but the destination is also popular among families with little kids.
Things to do in Railey Beach are cliff climbing, kayak tours and snorkeling.
Other off the beaten path destinations in the Phuket area are Phi Phi Islands, where "The Beach" with di Caprio was filmed, one and a half hours by ferry from Phuket; and Ko Lanta.