It depends which island(s) you intend to visit and spend time. Oahu is more like city, shopping, etc. I do not considered it the 'real' highway and as others have said Waikiki is where the action is and also some of the gay bars as well.
Maui and Kauai I have enjoyed and spent a couple of weeks in each, you can really get away from anything and get a sense of what is to just relax disconnect yourself from the mainland..get lost in the small local towns, but really great friendly locals, and you really get a sense of of what I call the real highway. Drink some really good Hawaiian coffee and also taste some amazing chocolate. For Kauai, the impressive Waimea Canyon is a must see, just like the Grand Canyon. Also is more lush than Maui in terms of flora, tropical than Maui which I considered a bit drier. Another, worth thing doing but is pricey is a helicopter ride with Blue Hawaiian Helicopter is an amazing ride and another point of sight in seeing the island(s).
Additionally, the private island of Niihau is available to go nearby for snorkeling, and whale watching by several tour operators which are available to take you, although you can not step into the island itself. Also plenty of sea kayaking and snorkeling operators as well which you can enjoy seeing dolphins and green turtles swim by. In the evening the amazing sunsets as well and if you wake up really early the impressive yellows, oranges of the sunrises. There are catamarans tours for sunset cruises with drinks that can take out as well.
I have not yet gone to the Big Island to compare, but others already have given their opinion. Anyway, have fun.