I presume your boat will dock on the east coast of this small island. You could take a cab or tuk tuk to the 'baan taling ngam' hotel for lunch at the beach club or the cliff top restaurant. Followed by a trip to Chaweng beach on the west coast for a spot of shopping (if you are not all shopped out).
Altenatively, there is a crocodile farm on the north coast with a crocodile show etc. depends on what you are looking for.

Ko samui is now a well developed (over developed ?) family touristy island.
enjoy the cruise.