Lucky you! Congratulations. 3-5 days extension isn't that long though, so it would probably need to be somewhere easily accessible from Phuket. You mention both Lanta and the (many?) Gulf islands. The weather patterns aren't the same on the Andaman and the Gulf side so knowing WHEN you're going would help.

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“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.