It depends on which island you're going to... If you're going to Oahu, I recommend the following:
* Pearl Harbor (Go there really early in the morning to avoid long queues)
* Downtown Honolulu (Ala Moana beach is awesome, where you can get the wonderful views of both the Diamond Crater and the city)
* Waikiki beach certainly (Dining there is pretty expensive, and crowded; I remember an Chinese/Italian restaurant called "Ciao Mien" in Hyatt Regency; I liked it, but it was quite expensive. )
* Polynesian culture center -- I didn't like it much, but it's a spot where everybody visits.
* Hanauma bay -- Famous for snorkeling.
* If you can spare one more day, I would HIGHLY recommend touring aroung the island -- you can either rent a car or just take the Circle Line bus. The views are awesome -- there's a beach around every 10 miles, and pretty in their unique ways. At least that was the part I personally enjoyed most. "A Ji Li Si" recommended last year before I went, and he was right.
That's about it. Hope this helps, and have a nice trip.
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