Buy the book, "Maui revealed" before you go, especially if you are traveling the road to Hana. It tells you every waterfall, scenic lookout, etc to stop at along the way. It was the best $7 we spent!
Old lahaina luau was top notch. ROYAL lahaina luau sucks. Don't waste your money.
Like Dave said, stay in the lahaina - kaanapali - Kapalua area if you can. The whole island is great though. You'll be in paradise no matter where you stay.
The drive up Haleakala was worth the 3:00am wake up. It is a beautiful site to see the sun rise above the clouds and realize how high up you are. Take a jacket, its cold! Didn't care for the bike ride down afterwards. You have to be part dare-devil.
If you want a fun night out, go see Warren & Annabelles in Old Lahaina. It sounds hokey when you read the description (slight of hand magic show with dinner and drinks), but it is absolutely hysterical! My wife and I were laughing for days. It will also leave you wondering, "how the heck did he do that!" Its not some big vegas, smoke and mirrors, make this elephant disappear show. Its more comedy than anything. We aren't big magic show kind of people, but that was memorable.
Have a great time! My wife and I are jealous!