It's difficult suggesting things for someone you don't know; but on Hilo, the "don't miss" is Volcanoes National Park, as part of a tour or by helicopter. On Maui, Haleakala Crater and the Maui Ocean Center are very popular. And Lahaina is great for just strolling around; touristy, but a lot of fun shops and restaurants right on the water.
Have a great cruise.
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