Two "musts" in Hawaii are Volcano National Park and Pearl Harbor. Beyond that it just depends on what you like--water sports, hiking, mountain biking, scenery, fly-overs, shopping;
it's all up to you.
I was on POA in Sept 2005. Did many excursions. Perhaps the BEST one was the sun rise at Haleakala Volcano in Maui. You have to get up by 3am but you will witness a sight that you will never forget. If you go, wear warm clothes and a jacket it is 10,000 feet high and very windy and cold!!!!
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Sovereign of the Seas, Royal Caribbean 11-17-03
Fascination, CCL 2-14-05
Fantasy, CCL 2-05-06
Elation, CCL 9-24-06
Carnival Victory, CCL 1-28-07
Sovereign of the Seas, Royal Caribbean, 9-24-07
Oh my gosh, I just went to Hawaii last summer for 5 weeks and there is so much to do. Parasailing was fabulous, swimming with the dolphins, the water park, just sight seeing- you will love. the submarine ride, snorkeling out with the dolphins - you will love it there. Have fun