Can't help with tours but we did something similar as a PP. We had a rental at Aulani and drove to the airport and took the first Hawaiian Air flight (5:00 AM) to the Big Island. Easy to pick up the car rental once we arrived (and easy drop-off). Then we stopped at Kens Pancake House right outside the airport. A definite "Can't Miss"! We drove to Volcanoes NP but honestly did not have a lot of time, since we had lined up a Lava Boat Tour for late morning. After that we had lunch, did a little sightseeing, then drove back towards Hilo for the last Blue Hawaiian helipcopter tour over the volcano. We then took an early evening flight back to Oahu. It was a fantastic day! We are just not tour people. We love the independence of going where we want, when we want. While we were on Maui for three days we did the sunrise on Haleakala (now THAT was an early alarm clock setting!) and we spent an entire day on the Road to Hana and back. As someone earlier mentioned, if you decide to make the day trip to Maui, you need to go as early as possible to have a chance at making the trip worthwhile, since the distances between places can eat up a lot of time.