If you only do one thing while on vacation (besides chill on Maui's many gorgeous beaches of course) make it the Pac Whale Honolua Bay Snorkel Sail. I'm not easily impressed, and I was just blown away by the whole thing. The snorkeling is beautiful. The animals are amazing--we saw turtles, a manta ray, schools of fish, and even a small shark. The staff is so friendly and helpful and patient. My girlfriend and I had her mom and my two aunts (all in their 60s) and they had an absolute blast both in the water and just being on the boat.
Then coming back down the coast with lunch and an open bar under sail is just magical. If I could give the whole experience 10 stars, I would. Stop reading and book your trip now!