Map of Koh Yao, Phangnga - Thailand
While Koh Yao and Koh Yao Yai are part of Phangnga, Province -- they are located in the bay between Phuket and Krabi. The island group does not seem to have the same appeal as Koh Phi Phi, but it is much more easily reachable.
Tourism has touched the islands and you can find some nice beaches and a really slowly paced lifestyle. The islands combined are roughly the same size as Koh Lanta and have a number of villages,
Getting there starts for Phuket's east coast -- Bang Rong Pier. Long-tail boats charge Bt 50 each direction and take about 1 hour. So you can easily scout the island as a day trip before making up your mind to stay or not.