Lots of information on this board about your question. Thinking about your situation, I would consider renting a car from the Waikiki area (assuming you are staying there) on the second day for a day trip around the island. If you rent a car the whole three days, then you'll likely need to pay for parking at night. On the other hand, you will be amazed at how quickly 3 days fly by. There is some convenience in having a rental car at the ready for getting to/from the airport, or where ever else you may decide you want to go.