How exciting! We just got back from Maui and the Big Island so I'm all blissed out. Here's some of what we did, FWIW.
Restaurants - we didn't go to a luau, I just couldn't get myself to pay that. We ate a lot of poke bowls and fish sandwiches - so good. Lahaina Fish Co was the nicest meal we had (it was good, and reasonably priced). Joey's Kitchen was fantastic for takeout, Down the Hatch in Lahaina and Castaway Cafe were both really good. And we actually planned to go to the Honokowai Okazuya & Deli that georgiegirl mentioned, but we ran out of time! Must go back.
We took a catamaran trip with Gemini. The crew, boat, lunch and snorkeling were all fabulous (we spent our luau $$$ here instead and didn't regret it). It was a very smooth ride, even though there were big waves and loading from the beach was kind of precarious. But I have no idea if that's always the case.
If you plan to snorkel, consider signing up for The Snorkel Store's daily email report. We found it really useful. Kapalua and north of that was all stirred up while we were there, but Airport Beach was the best snorkeling we did from shore. Black Rock was fun too, but didn't compare to the reef at Airport!
Haleakala - half a day would have been plenty unless you plan on hiking far into the crater. Kula Botanical Gardens on the way there or the way back if you're into that
DD and I went ziplining, and we went on a chocolate tour at DS's request. They were both great experiences. We didn't drive the Road to Hana because we'd just come from the BIg Island, where we saw a lot of waterfalls and spent a LOT of time in the car. Otherwise I think we would have.
If you plan to drive around the island much, we really like the Gypsy Guide app. I think the Shaka app is a similar idea.
Hope you have a great trip!!
-Christine
DD, 9/07
DS, 5/09
and a very tolerant pup