Aloha! I'm in Maui right now (first visit) and am smitten with the island.
I have been to the luau at the Royal Lahaina, as it was the one recommended by many, and it was very good but not outstanding as far as these cultural shows go. Once you are in, you can imbibe to your heart's content. The setting is beautiful and the performers are very professional. They change costumes very quickly between sets! The food is good and I especially liked the pulled pork roasted underground.
I took a snorkeling excursion on a 60 year old catamaran to the Molokini crater and the Turtle Island. Very fun, but my back got sunburned. I recommend this or another similar excursion, but perhaps wear a Tshirt. You can ask for a wetsuit but the water is warm enough not to need it. These boats leave from Maa'alea Harbor. At the harbor there is a neat little museum (free) and a wonderful pizza restaurant called Porto. There is also an Aquarium, which I have not visited.
Today I had a wonderful day. I started by driving to the top of the Haleakala crater - 1.5 hours of switchbacks - and the changes in climate and geography were amazing. You start in the tropics, go up through temperate climate with dairy farms, and end up climbing scary, unfenced hairpin bends in volcanic tundra at 15 mph through the clouds. The elevation at the top is just over 10,000 feet, and I definitely was conscious of my breathing. At this point you are either in or above the clouds. When I arrived, the sky was blue and I could see a volcanic peak on another island, but while I was there the mist rolled in and my trip down the mountain was like driving through pea soup fog in Newfoundland! On the way down I visited the Lavender farm, which is not yet at its seasonal best, but is lovely. Then I noticed a sign for "Historic Makawao Village" which looks like something out of the old West but dates from the 1940s. It has become an artisan's haven. On reaching the lower part of the island I headed for Ioa State Park, which is a tropical oasis in a sheltered valley. On the way back to Kihei I stopped at Maa'alea Beach for a walk through the water and I still have sand between my toes. There is nothing like being spontaneous!
All these places (except perhaps Makawao) are worth a visit if you have the time. I have rented a car and brought my own GPS, which is very helpful. The maximum speed limit that I have seen here is 55 mph and many sections of road are signposted with speed limits that appear excessively cautious......and that many people ignore. I keep close to the speed limit but impatient drivers overtake and honk at me!
Beaches abound on Maui. I do not presume to know the best for snorkeling, but all of the sandy ones are wonderful for walking. In general, the water seems choppier in the afternoon, so snorkel trips begin in the early morning.
I am here till April 30th and will be visiting Oahu next week. If you find my ramblings helpful, PM me at the end of my vacation when I have seen and done more!