Hawaii is one of the 50 states in the United States of America. United States currency as found on the mainland is the legal tender of Hawaii as well. If you are an American traveling to Hawaii, you therefore do not need to exchange currency.
Some banks do and others do not. Depends on the bank and the currency. When I got back from Russia I still had about 500 rubles and wanted to exchange them at my bank, but they wouldn't take them....
Hawaii uses the American Dollar. Because it is an American State, it is only logical that it would use the same currency as the rest of the country. All states, such as Virginia and California, use...