Try to find someone to take you to the ruins on your own and avoid the costly ship tour. You can also avoid the ruins as not that big of a deal to see. One hour to get there and one hour maximum to walk around the site.Then another hour to drive back to the ship.
No cruises added
Hire your town taxi driver and take a tour around the island. Go to Money Beach for snorkeling.
Don't bother walking into town. Nothing there. NCL is building their own port nearby.
Try to stay on the ship on of the port days or get back early to enjoy the pool area and lunch without the crowds.