I will arrive HNL at 7:30am (from NRT) and depart at 6:30pm. I am thinking about:

1. Rent a car, drive around the island, stop at a beach and swim a a little bit and go back to the airport.

2. Use my frequent flyer miles to get an intra island ticket to another island and come back.

The first option, I can do that if I can find a place to take a shower as I don't want to get a hotel room just to take a shower. Is that any public shower at any beach?

Actually I like the second option because I have been to Oahu and Hawaii islands already. I would like to visit another one if time permits. However I have only 11 hours, after clearing immigration and custom and I have come early before the flight departs (although I don't have to check in for the evening flight because it will be done in NRT). Any suggestion is appreciated.

You probably don't have enough time to travel to a neighbor island. Even if you did fly away to Maui or Kauai, you would have less than six or so hours and then have to come back. IMHO not worth the $125+ to spend on a RT ticket.

There are public showers at most popular beaches. Just head down to Waikiki Beach or Ala Moana Beach Park. Both beaches have shower stalls. Even Kailua Beach Park and Lanikai have showers as well (on the Windward side). Renting a car would be good, or you can take a shuttle into Waikiki for $13 roundtrip. Just inform the shuttle company where to pick you up.

http://www.state.hi.us/dot/airports/oahu/hnl/hnl_ground_trans.htm

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