Costs in Hawaii are always higher than on the mainland (transporting everything across the waves can be hideously expensive), but there are two high seasons (winter and summer), with the two shoulder seasons being spring and fall. There is NO low season here.
Weather
It's spring all year round, with constant temperature in the 70s. (You could, of course, climb to the summit of Mauna Kea and get really cold up there, if you insist!)
Activities in Hawaii
With the temperatures of Paradise year round, most people who come here want to be outdoors as much of the time as possible. This is the place to go surfing, kayaking, boating, swimming, scuba diving, snorkeling, hiking, biking and hang gliding, you name it, Hawaii has it. (In Maui, there are even genuine submarine rides.) For the indoor activities, start with the Polynesian Cultural Center (run by the Mormon Church), and be sure not to miss a visit to the USS Arizona at Pearl Harbor. For real art lovers, try the Bishop Museum in Honolulu.