The Bus works pretty well in the Honolulu area for relatively short trips (e.g. Waikiki or Ala Moana area to Pearl Harbor or Diamond Head). It can be used for the trip in question (#C express or #40), but it will take about an hour. Renting a car for the day (and returning it without having to park it overnight) is a good option for this trip if parking at the destination is not a problem.
I would probably rent a car myself for something like this. But, counting check in/check out the car, plus driving there, not much difference than taking The Bus, at least in regard to time.