My one afternoon trip
I don't know how long you're in Okinawa but I only had one afternoon of work flying there from BOS.
Here's what I did: I arrived on the late NRT to OKA flight and took a cab to the Double Tree (1100 Yen). The next morning I took a cab back to the airport to pick up my rental car (1000 Yen). You have to wait outside next to the flags for the rental car company to pick you up and take you to an off site location. When it was time for me to leave the next day. I dropped the car off at the rental car company and they took me back to the airport for my flight out.
Tip: Buget was the only rental car that didn't have a three day advance notice for renting so I had to go with them instead of Hertz (Toyota).
Also you may have to call the rental car company to come pick you up. the numbers arre on the flags. A Japanese lady from another company called for me.
If you get to the hotel hungry there is a great Noodle shop open after midnight about a 1 minute walk out the back door of the lobby to the right on your right. its easy to miss becuse you might think you have to walk further as I did.
Good luck.^