There's a lot to do in Maui, regular tourist stuff, and then there's the active water sports. The north shore features windsurfing, surfing, stand up paddle surfing, canoe racing, scuba, snorkeling, bicycling, kayaking..you name it.

The south shore has gentler surfing beaches, kayaking, snorkeling, surfing and lots of places to swim.

You can also just watch some of the world's best surfing and windsurfing atheletes, up close and personal at places like Ho'okipa and Honalua Bay.  

If you are the active traveler type the Maui Trailblazer guidebook is recommended. It has all the excursions possible listed in a clean and organized format.