For destination couples, their families and guests I have a real find! Hawaii 23 Transportation. I have used them several times and they are hands down the best value on the island for getting back and forth from Honolulu International Airport and Waikiki.
The cost is just $23 dollars - less then the airport shuttle for 2 people and direct service to your hotel. I highly recommend checking them out if you need a ride:)
http://www.hawaii23.com/