I would not miss the Haleakala experience even if you do have to wake up at 2 a.m. for the drive in. Makes for more snuggling up together time in the dark and cold while you wait for the sun to make an appearance. The drive down is beautiful as well. Hana is a nice, quaint place but the drive will make you queasy. Another alternative for "cruise" around the island is to visit my Auntie Lorraine (not really my aunt, but that's just Hawaiian style) at her shave ice (translation: snow cone) stand out in Kahakuloa. It's way out past Wailuku or past Kaanapali if you go the other way around--pretty much where the rental car companies tell you not to drive (but there IS a road all the way) so why not? Just go for a drive and enjoy the scenery then stop by and have a cool shave ice treat, maybe some banana or mango bread and relax on her back porch on the bay.
Maui is beautiful, but is getting way to urban for my tastes... although I live on Oahu and grew up here, we used to go to Maui to get away from the hustle and bustle....but not anymore. Still, it's a nice place to visit. The Maui Ocean center is pretty nice if you just want to see marine life up close and personal as well. A 1/2 day trip on a catamaran to go snorkeling at Molokini is great too. You can even get a video of the event. Some captains have crew that will fish right off the boat and catch your lunch--fresh mahi mahi anyone?