Honolulu and its Waikiki district on the island of Oahu is usually the first place to visit in Hawaii. I was equally interested in the night life, beaches and nature, and Oahu offered the first two of them. For nature you'd better head to the other islands in Hawaii. But the beaches are great, and the nightlife is quite good. There are also good opportunities for expensive shopping in Waikiki. The weather is great all year long, the atmosphere is very laid-back and relaxing and the locals are friendly. Of course there are tourists everywhere, many of them from Japan.
The opportunities for watersports are great. The water is very warm so that you can swim as long as you like. If you like to increase your risk of getting melanoma, there are plenty of beaches with very good views. I did some snorkeling in Hanauma bay and saw lots of colorful fish but unfortunately no sea turtles. I also burned by back pretty badly, so I advise to wear a t-shirt in water too. You should also try surfing, there are several instructors available.
To clear up hangover I climbed on Diamond Head, which provides a wonderful view over Waikiki. After reaching the top I found out that the new memory card for my digital camera didn't work.
Downtown Honolulu has quite a few interesting places to visit. I saw 'Iolani Palace (but didn't have time for the full tour), Chinatown (pretty much like any other Chinatown), Maritime Center (quite nice seafaring exhibits), Aloha Tower (ok view), state capitol building (as interesting as you can expect) and Foster Botanical Garden (be sure to attend the tour). There's still some places I wanted to see but didn't have time, such as the Bishop Museum.
To see other parts of Oahu you pretty much need a car. I rented one for a day and visited the Arizona Memorial in Pearl Harbor (one of the places you have to visit even though there is not much to see), Dole's pineapple plantation (everything you wanted to know about pineapples) Halei'iwa and its surf museum (better in winter), the Polynesian culture center (nice shows), Byodo-in Temple (beautiful) and Nu'uanu Pali Lookout (good view, was very windy).
Hawaii is to Americans what Canary Islands is to Europeans. They are similar in nature and climate, but Hawaii is a bit more upscale.