If you're in Maui for the whole day, I would suggest renting a car - that's what we did. We explored the island, hit a beach with the locals in a little town, found a botanical garden on some road going from here to there, found a great place to have lunch in another little town, went to IO State Park and found a lovely oriental garden on the way . . . you get the picture... If you're docking in Kahalui, there will be shuttles waiting at the pier to take you to the car rental places at the airport; if your docking in Lahaina, I think you'll need to pre-reserve your car so that the shuttle can meet you there to take you to the airport. Either way - it was a great way to spend the day on Maui.