I just got back from Maui last night, my 5th time there, by far my favorite island. Maui has enough night life to satisfy younger people, but not as many people so that you feel overwhelmed by traffic on the road or too many people on the beaches. We went on this short waterfalls hike and tour of west Maui which was beautiful.
But in Honolulu, hike Diamond Head, see Pearl Harbor and hit up the International Marketplace. There are plenty of beaches, restaurants and clubs that you'll stumble across.