Although HNL is not the best of airports, a 4 hour layover isn't so bad! But... 8 hours is probably great. Either way, i'd consider leaving the airport for a bit. Pearl Harbor tour is great if you havn't done so already - and close to the airport. For a nice lunch in the warm afternoon sun, try Kaka'Ako Kitchen @ Ward Center in Honolulu.
Transportation wise.... "THE BUS" is a great transportation option island-wide, provided you have TIME to spare. It's like 45 minutes from airport to Waikiki I believe. Consider a rent-a-car for the day?
Have a blast =)