In Hawaii full public nudity is technically illegal, however there are several beaches where that technicality is overlooked by patrons of the beaches and enforcement.
There have been enforcement efforts in the past, and it has been ruled that exposing breasts is not considered nudity. However, there is a significant portion of the local population who are offended by nudity, including bare breasts, so if you plan on popping the top it would be best to do it where it is expected.
Little Beach – No inhibitions here! By far the most well known (and populated) clothing optional beach – in Makena (South Maui). Little Beach Page
Red Sand Beach – Once a place where most got nude, now that visitors have found out about it you’ll probably have plenty of clothed company. In Hana. Red Sand Beach Page
(Paia) Secret Beach – A lesser-known secluded clothing optional beach on the North Shore known by locals simply by this catchall name (there are many beaches called “secret”.) Paia Secret Beach Page
Others – There are a few other secluded beaches and coves scattered about, and some, like Baldwin Beach Cove where topless sunbathing is common, but full nudity is not. You may also discover nude bathers in the jungle if you hike to more remote pools and waterfalls. The upper pools at Twin Falls also periodically has folks swimming in their birthday suits; the owners of the land are said not to take issue with this.