There is so much to do and it really depends on what you like. I second the Maui Revealed recommendation.
If you are a beach person, I recommend a trip to Big Beach (also known as Makena, also known as Oneloa) but mostly known as Big Beach. It is about a 50 min. drive from Ka'anapali as it is south of Wailea but for us, it is worth at trip at least once each visit because:
1) It is almost always sunny there, regardless of what it is like anywhere else
2) It is drop-dead beautiful
and
3) It is the largest, untouched by development, white sand beach left on Maui.
A word of warning: Next to Big Beach, over the little lava wall to the north of the beach is the way to Little Beach, which is clothing optional.
Another word or warning: Don't leave anything in your car at any beach. It will be stolen
A last word of warning: The waves at Big Beach can be bigger than your average beach wave at certain times of the year. Be careful. They can hurt you if you're not careful but they are a blast if you are careful and know what you are doing.
We also like a trip to the Upcountry because it is such a different side of Maui from the beach/resort scene.
Snorkeling at Black Rock, which is north of the Marriott (walking distance along Ka'anapali Beach) is good.
For us, a burger at Cheeseburger in Paradise in Lahaina is a tradition.
Hope that helps.
Bob