My absolute Number 1 suggestion to you if you want to do something outdoorsy (aside from sitting on the beach) is to go snorkling on the Alii Nui. The two times that my wife and I have been to Maui, we've gone on this boat. It's a catamaran that leaves every day around 8 am from the marina that's half-way between Wailea and Lahaina (I forget the name of the marina). Instead of taking you to Molokini Crater where about 2,000 other people will be snorkling, they take you to a secluded spot (not far off-shore south of Lahaina) where you will be the only snorklers. It's a half-day trip and it cost us about $60 a piece. Well worth it. We saw tons of fish, including flying fish. We also saw a bunch of sea turtles. The boat only takes maybe 20 people so it's much more relaxed than one of the boats that takes 50 or 100 people. They serve food, drinks, etc. We really enjoyed it. Also, take the road to Hana. Leave early. Don't do the sunrise bikeride down the mountain -- unless you like getting up at 1:30 a.m. on a vacation.