Wind surfing, or sailboarding, is probably best learned on the Windward side of the Oahu. In the suburban town of Kailua, you'll find several surf shops with equipment and instructors. On Maui, take at look at the activity at Ho'okipa Beach, one of the most famous wind-surfing locations on earth. Accomplished sailboarders from Europe pine for the day they'll hit Ho'okipa. Whether by bus or car, get away from your hotel from time to time and see what else Hawaii has to offer.
Rental cars are ubiquitous and highly competitive.
If you choose carefully, you'll probably find them much less expensive than comparable cars on the U.S. mainland. You will also find a number of hiking opportunities, some of them run by commercial entrepreneurs and others you can do by yourself. A simple Honolulu hike almost everyone can do is the gentle climb to the top of the volcano named Diamond Head. On all islands there are various daytime and evening cruises on boats ranging from rubber Zodiacs to multideck excursion vessels offering sit-down dinners followed by entertainment and dancing. Most popular are the diving and snorkel tours during the day, and the moonlight and starlight cruises in the evening.