Where on the Island are you gonna be?
Kaanapali/Lahaina? Black Rock in front of the Sheraton has some of the best snorkling I've experienced. Deep clear water. Huge turtles. Giant plates and pocci's. Awesome!
Wilea? Makena aka "big beach" and little makena "little beach" has some really good spots, but depending on time of year it can be kinda rough. Most beaches off of Wilea have reef right out front. We had a killer reef I swam every morning, right out our front door!
Molikini Crater/Lanaii: I'd recommend going with the Pacific Whale Foundation. There's lots of snorkle boat trips that give you beers and burgers. BUT, everyone on the PWF boats has AT LEAST a bachelors degree in marine biology. Most have post grad schooling as well. Very nice, very informational.
I've heard some people say they weren't that impressed with the crater, but everyone in my party really enjoyed it.
Lanaii's snorkling was out of this world. Crazy under water topography and wild life.
I'm jealous. I went twice in 6 months and don't know if I'll ever get back, but we tried to do everything touristy in those 2 trips. Such a diverse Island. I think they shut down the 32mile downhill bike rides down Haleakela volcano, but you will never regret going up there to see a sun rise. It was like being on top of the world. Most spectacular natrual phenomenon I've ever witnessed.
I also highly recommend doing one of the Zip line tours thru the mountains. Great fun. PM me if you want other tips on where to eat. We experienced a ton of great places. MY brother owns a place there so he hooked us up on where the locals go too.
Lastly, don't forget to stop at Tropix surf shop right by the airport. That's where "Maui Built" apparel is sold. Only place in the world you can get it. No catalogues, no internet sales. Something that will give you cool bragging rights.
Sorry for the brain dump, but that place is the best. LMK if I can help you out.
Cully