The Haleakala Zip line tour is alot of fun and not very scary. I found the Crater itself to be pretty boring.
The beach at Ka'anapali in front of the Westin has great snorkeling or you could walk down to Black Rock in front of the Sheraton. We did Four Winds also- it was ok for the boat ride, but the snorkeling was much better right off the beach.
The Pacific Whale Foundation Whale Watch was reported to be great by my family- I did not do that. They said the whales came right up to the boat- contrary to what the PP said, so it probably depends on the day.
The drive around the top of Maui is great. You get to see a big blowhole- we played in this for hours. It is a "dangerous" ride and will violate your rental car agreement- we did it anyway- no problems... except for the time we had to pass a construction vehicle and my SO's side of the car was hanging off the side of the cliff, but hey- it's an adventure.
The Road to Hana was my favorite thing. Beautiful ride- make sure you take the time to get out of your car and explore. The CD from Boss Frogs is helpful.
Wai'anapanapa (sp) State Park- Whoa.... behold the beauty. So much to explore here- lava tube, black sand beaches, freshwater caves filled with princess legends, a really old cemetary, trails, strange trees, lava formations, sea arch...
Iao Valley... another beautiful spot- combine this with the drive over the top of Maui.
Also- many of those tourist guide books have a coupon for Boss Frogs. Rent a snorkel set for the week for $9 and get a free afternoon snorkel trip on Four Winds. Rent a set for everyone and you all get a free trip- that way you have gear for the week to snorkel from the beach and you get to try a boat ride, although the AM boat rides are reportedly better. There is also a $7 ticketing fee for your "free" boat trip. This is still cheaper than paying for the trip and you have gear for the beach.