Ko Muk is the Trang island with one famous trick and a lot of quiet charm around it. The trick is Emerald Cave, a sea cave that opens into a hidden beach inside a collapsed limestone chamber. It is one of the best day trips in the Trang islands, but Ko Muk is better if you stay long enough to see it after the tour boats leave.
The island is small, hilly, and still partly a fishing community. The east side has the pier, the village, and the long sand spit around Sivalai Beach. The west side has Charlie Beach, the sunset beach and the usual base for kayaks to the cave. Between them runs a narrow road through rubber trees and simple houses.
Ko Muk is not polished like Phuket or busy like Phi Phi. That is its appeal. You come for a few days of swimming, kayaking, seafood, and island walks, with the Emerald Cave as the set-piece excursion. In the dry season, boats link Ko Muk with Trang's mainland piers and nearby islands such as Ko Kradan and Ko Ngai; in the monsoon months, transport is more limited and seas can be rough.