MUST DO :
If you are going this time of the year, Humpback Whales are wintering off the coast. You can watch them forlick in the water. Best places to watch are along the west coast. Remember, Maui used to be a whaling town. You can still see lots of the history at Lahaina - the town between Kihei and Kaanapali.
1) Haleakla - do the sunset part, not the sunrise. Just drive up there, no need to hike the trail as you will see the Moon or Mars like surface just from the National Park office which is situated at the top - further up there is observatory which is restricted to visitors.
2) Go to beach areas for snorkeling or whatever beach activities you like - our personal choices are beaches at Wailea / Makena areas (South part). No facility at Makena Beach (Big Beach) but it is the most beautiful beach in Maui. Further down, after the end of the road, and hike a short distance, is the Little Beach where clothing is optional, but the beach is very nice also. All beaches in Hawaii are public beaches and the hotels have to provide public access. You can go to beaches fronting Grand Wailea / Four Seasons Resorts but they are quite crowded. Grand Wailea has a very impressive reflection pool. The time we went it was all decorated with orchids, unbelievably beautiful. The pool is kid's heaven but only guests can use it and they STRICTLY ENFORCE it.
Kaanapali also has long stretch of beach (West part) The portion fronting Westin is the best part. "Black Rock" fronting Sheraton is the part to do snorkeling. Hyatt has a very beautiful ground and you can see Flamingos walking around, very serene.
Kamaole Beach Park I, II & III at Kihei (South part, just North of Wailea) - probably is the best bet for family with kids, especially Beach I. Very wide beach with calm water and ample parking areas. Even the obnoxious Maui Revealed guidebook recommend these beaches.
Caution about Molokini - only take boat tour if weather is good and water is calm. Otherwise, you would have a very rough ride and you may get seasick so much that you dont even want to go into water. And if sea is very rough, you could hardly do any snorkeling.
3) Take a drive to the up mountain area at the South side - pleasant scenary along the way and you will be rewarded by the aerial look of Molokini in the distance. Easy drive. Wine tasting toward the end of the drive (dead-end hway unless you have a 4wd then you can loop over to Hana and drive The Road to Hana back to Maui downtown.
4) Paia (spelling ?) - one of the most famous windsurfing spots in the world. On a windy day you will see many colorful windsurfs flying accross the water.
5) If you are a good driver and are very adventurous - try the drive of North Shore, Hwy 340. NOT RECOMMENDED if you have small kids and/ or are afraid of height. The road is very narrow, one side is sheer cliff going UP, and the other side is sheer cliff going DOWN (to the ocean). The view is extremely beautiful but the road is also extremely difficult. Only go if you have good weather for at least a week before you go (so no slippery spots) and you have a good navigator (the person at the passenger side has to watch for oncoming car). And drive from west toward east, so you stay at the mountain side, instead of the cliff side.
You should have a great time.