Just got back so here are some good things to occupy your time:
Zip Line - http://www.skylinehawaii.com/ - The guides were telling us that another spot was going to open up near Lahaina
Whale Tour - http://www.pacificwhale.org/ - Non-profit group. Excellent tour. Bring your high end SLR as your pocket digital will probably not be able to catch a whale breach fast enough. Binoculars are nice depending on where the whales are at.
U'lalena - http://www.ulalena.com/ - An offshoot of a Cirque performance with some Hawaiian flair.
Aquarium - http://www.mauioceancenter.com - Small aquarium but very impressive. Good rainy day stop.
The road to Hana - Once you actually hit the road (this is where it becomes very twisty), travel about 10 miles in, then turn around. You'll predominately see very similar things over and over and over and over. If you do decide to go, take your Dramamine - 613 turns in a short distance as I recall. A motorcycle (cruiser) is actually better than a car as the road gets very narrow in spots.
Paia - local town on the east coast. Just south of town is a spot where the windsurfers hang out. Very cool to watch. Charley's is a good stop for food and drink.
Food spots to avoid - Gia Don's (Kihei). Very poor service, expensive for Italian out of a can.
For better food options, get out of the tourist zone and find some restaurants off the main strips.
Pineapples for the trip home - http://www.mauitropicalplantation.com/ - Prepackaged pineapples in 1, 3 or 6 pack boxes already inspected and packed to be checked.
Alcohol runs - Beer is cheaper at Costco near the airport. Pick yours up on the way to the hotel. Costco is also a good spot to pick up bulk boxes of the Maui Carmacs that you're sure to become addicted to while there.
The Hawaii Super Ferry - http://www.hawaiisuperferry.com/ should be running by the time you get there as well. Take the rental over to the Big Island (if feasible) and roam around Volcanoes National Park. Best spot at sunset is a walk out on the lava flow (where the lava meets the ocean. Bring a fleece and a flashlight as it gets cold here at night and the footing isn't great. -----