There is a ton to do on Maui.  There are numerous sites on the web that talk about all of the different activities including:

• Fodors Main Page:
• Fodors Top Picks:
• Maui Activities: 
• Maui Activities.net:

Our recommended activities are (click on the activity for more info):

Take the Scenic Drive to Hana:   http://www.alternative-hawaii.com/special/mns.htm

Go watch the Windsurfers in the waves at Hookipa Beach (Road to Hana)

Go to the Spa at the Hotel:

Play Golf:

Go Scuba Diving:  http://www.maui-scuba.com/

Catamaran to Molokai or Molikini:

Go Snorkeling Nearby:   http://www.snorkelbob.com/sbfrmst.html
Get you gear at Snorkel Bob's in Kihei and go to Ahihi Bay Marine Reserve

Drive the scenic north shore past Kanapali & Kapalua:

Take a Helicopter Tour of the Island:  http://www.bluehawaiian.com/

Go Fishing:  http://www.fishmaui.com/

See the Sunrise (if your really lucky) on top of the Volcano Haleakala