You really do need a rental car in Hawaii in order to leisurely explore the Hawaiian islands.  So, I just learned about this “free” rental car deal offered by United Vacations.  The LA Times travel blog reports this deal as follows:

Purchase a five-night/six-day vacation package, including a round-trip flight and hotel stay on any island, with United Vacations. Enter the promotional code SAVEBIG when booking online, then add on a five-day economy car rental from Alamo for free (note: the small print says that local taxes, incidental charges and optional insurance are not included, so the car rental is not 100% “free�). Still, it’s a good deal, especially since Hawaii vacation packages totaling over $1,200 qualify for an extra discount of $50 to $200.

The post goes on to advise that you do need to verify that you are actually getting a deal before booking as they found some packages that ended up costing more than booking the hotel, flights, and car separately.