Lahaina - Front Street is very cool - lots of ocean front restaurants, shops, galleries etc. Many nice beaches nearby. Haleakala is about a 45 minute drive. Harbor has many fishing, snorkel/dive charters. Whale watching December thru April.
Kauai - not sure where Nawiliwili is, we stayed southeast shore near Poipu. Best thing is Waimea Canyon, or a helicopter tour to see more (NaPali coast etc).
Hilo - Volcano. Mauna Loa macadamia nut plantation/factory is close by.
Honolulu - Pearl Harbor is a must. Diamond Head & Waikiki are cool. Hanauma Bay, as people have said, is some great snorkeling.
Kona - This area is becoming more developed. Kahalu'u Beach Park has good snorkeling with lots of turtles.
Unfortunately having to be at sea in the evenings you won't get to do a luau.