You should be able to do Pearl Harbor......generally there is a pretty long line to take the boat over to the USS Arizona monument...but you'll be getting there pretty early in the day. Other than that....Waikiki Beach is a nice place to kill some time. After being on Lanai.....it will seem like Grand Central Station though but better than sitting at the airport. If you are feeling energetic....catch a cab to the Diamond Head Park and take the trail up (but perhaps you won't want to get all sweaty before getting on the plane). You don't really have enough time to get to the North Shore. We did something similar inbound into Kauai year before last but had a little more time than you'll have.