The road to Hana is a must to really experience the true beauty of the island. Lots of flora and fauna.
Sunset sail on true sailboat---beautiful.
Snorkel molikini
Bike down Haleakula (dormant volcano)
Go to the Maui Tropical Plantation--see lots of fruits, coffee, and beautiful flowers.
Iao Needle, state park, beautiful
Quick Tips:
Eat at Mama's Fish House-expensive /pp, worth it, the scenery and uniqueness of the food was wonderful.
Best area to stay in Kaanapali area, most hotels are perfectly fine. Do NOT stay in Royal Lahaina Resort, that is where we stayed, very disappointing. Old, not well maintained.
Do drive all the way around the island---yeah I know your rental car contract says not to but do it anyway, it is oh so worth it, and everyone does it anyway.
Visit Upakula area and Tedischi Vinyards, and be sure to sample some pineapple wine.
Best Way To Get Around:
Get a rental car. Make sure it's a compact, believe me that is what you want on some of the very narrow roads around the island.