Snorkeling: This is a must. There are lots of fine snorkeling spots on Maui, but here are two of my favorites: Snorkel off of Black Rock on Kaanapali Beach right in front of the Sheraton. It's safe and VERY CONVENIENT. Try it (WITH A FLOATATION DEVICE AND A FRIEND!) even if you don't swim well. The best views here are at the end of Black Rock in the natural cove but be careful of the currents and waves.
For a somewhat better and more exciting experience, take a ride out to Molokini Crater. Several tour companies can take you there daily. I happen to like Trilogy.
Day trip to the Island of Lanai: This island is a great place to stay for a few days. Both hotels are 5 star. But if you don't have the time, try this day triip: Take the morning Expeditions Ferry from Lahaina harbor to Lanai. Purchase an all day trasportation ticket when you get there. Start out by exploring the Manele Bay Hotel. The grounds are spectacular. Consider having breakfast there too. The buffet is world class but expensive. After you've taken in the Manele Bay, take the shuttle up the mountain to the Lodge at Koele. This too is an outstanding property but totally different from the Manele Bay. Explore the grounds and have lunch there. Hop on the afternoon ferry back to Maui.
LaPerouse Bay: This is a beautiful, historic ancient Hawaiian fishing village. There's not much left of the village, but the scenery is beautiful. Take a stroll along the seaside path too. The drive there is half the fun. Take the road past the hotels in Wailea. Keep going to the very end (you'll know!) Along the way, you'll pass thru a spectacular lava field from the most recent eruption on Maui....200 years ago, more or less. Stop in at the Four Seasons Hotel on the way back. Give the valet parking guy a few bucks and enjoy exploring this beautiful property. Have a lobster salad sandwich at the pool side, beach front restaurant.
The drive to Hana: Ok. Let me give you the straight skinny here....in my opinion, that is. The drive is safe, scenic and interesting. Yes, there are lots of curves and one lane bridges, but the road is good and there is no danger. Take the "I survived the road to Hana" T shirts with a grain of salt. The drive and scenery along the way are the best part. When you "get there," there isn't that much to see. Rather than going via the conventional tourist route and returning the same way, try going the "back way" and returning the touristy way. You will essentially wind up driving around Haleakala. Take the road past the Tedeschi Winery (stop in as you pass it). Keep going through some spectacular scenery. Stop at the Ohehu Gulch (used to be called "the seven sacred pools." There not seven, they're not sacred and they're not pools. But it's a nice National Park. Keep driving toward Hana and take a look at Charles Lindburg's grave. The location is not marked, but you will pass a sign for a church on the right side a few miles before Hana. Drive a few hundred feet along the dirt road and park at the end. It's worth it. When you get to Hana, stop at the Hotel for lunch and a clean bathroom. Continue on along the main road. Take a look at the black sand beach and caves at the State Park. Wear some insect repellant if you walk down to the pools.