Got engaged on Maui in Oct '03, so my info is aged but consistent with a lot of what has been said. Get the convertible. We stayed in Lahaina for the central location. We did sunrise on Haleakala but not the bike ride as my wife wasn't much of a rider. Do it early in your trip as being on Eastern Time will help you get up early enough to make the trip. The drive up alone is worth it. It will be cold at sunrise, but warms up quickly, so bring layers. We did hike the crater after too, and I recommend that. If you're really adventurous, you can hike down the crater to Hana. We didn't. I think that's a two-day hike. And the Uber back up to get your car would cost a mint.
It was quiet in October when we were there, so the Road to Hana wasn't very crowded. It's a must-do. As others have said, start pretty early. We took lots of little excursions down side roads along the way. Someone said not to drive and to take a tour, but forget that unless you're a lousy driver. You'll drive in and out of rain on a twisty road, but it's not as nerve wracking as a lot of drives I've done. The rental car companies all had restrictions on going around the south end which is why most drivers go back the way they came. The road is supposed to be treacherous, but we had no problems in our Chrysler convertible.
The surf and snorkeling are just fantastic. I heard Molikini was damaged. We took a snorkel trip there which was amazing. Snorkeling among the sea turtles is just incredible. But you can walk in off most beaches and find great snorkeling.
After a week on Maui, we moved on to Kauai where we stayed in Poipu for the swimmable beaches, warmer water, snorkeling and for more restaurants. "More" is relative, as Kauai is quite quiet. Our hotel's pool closed at 6:00, for example. And lots of restaurants closed on the earlier side. The north side resorts like Princeville are beautiful, but the water isn't swimmable much of the time with the surf and currents.
You think Maui is incredibly beautiful until you get to Kauai. The boat tour on the Napali coast is phenomenal aside from sea sickness (about 75% of the people on our catamaran hurled in 6' swells). Our captain took us right into a waterfall coming down from a couple of hundred feet up a cliffside. So cool. And the helicopter tour is well worth it too. Just try to get the best weather day for it that you can as they may not be able to get you into some of the ravines.
There is hiking galore. We took the road around the north shore all the way to the end for the trailhead to Kalalau Beach. It's about a 2 hourish hike to the beach. If you start early enough, you can then take the trail inland along the stream up to a series of amazing waterfalls. Just leave enough time to get back. We finished the hike in the dark (shorter days in Oct) which wasn't fun as you're along cliff edges on a narrow trail at some points. If you do this from Poipu, start early and drive directly to the trailhead without a lot of stops on the way. Use another day to go back if you want to explore the civilized part of the north shore.
Wai'mea Canyon is also incredible. It can be a half day if you just drive up and explore the rim or much more if you want to hike that area.
I think someone else mentioned Barking Sands Beach for sunset. Rent a Jeep if you go or bring an extra 6-pack to reward a local for winching you out of a rut if you get stuck.
Whatever you choose, have fun. It will be an unforgettable trip.