Maui is the best! That's what Maui No Ka Oi! means. There is so much to do you will be wet all the time - from jumping into fresh water and salt water to sweating from heat and exertion, and from peeing your pants with glee.
Highlights for the back door adventurer include getting to know some of the many water falls, cliff diving into the ocean, spear fishing, hiking the crater then staying in one of the cabins, camping, checking out the Redwoods, lava tubes, local tropical foliage and birds, climbing coconut trees, rain dancing, bathing in the sunset, and witnessing the phenomena of the Haleakala sunsetting with the full moon rising.
For the front door traveler there are an abundance of planned activities such as sunrise at the crater and biking down the 10,000 feet of the winding Haleakala road into the quaint town of Paia. Try surfing, wind-surfing, and body boarding for the first time with trained professionals that will keep you from drowning or getting bit by a shark. Don't forget the lush beauty of The Road to Hana stopping at the Seven Pools and then the historical Kaupo Store in the middle of nowhere.
Quick Tips:
The Old Lahaina Luau has excellent Hawaiian performances with authentic Hawaiian cuisine, well worth it.
The mystical theatre piece, "Ulalena", in Lahaina will give a musical history of the Hawaii.
The Maui Arts and Cultural Center in Kahului offers quality films you'd see in New York.
The Maui Farmers/Arts Market Friday mornings in the Kahului Queen's Mall has the local fruit at local prices - an abundance of pineapples, papayas, mangos, bananas, avocadoes and strange fruits you don't know how to open, as well as local arts and crafts.
Go to Makena, the most beautiful long stretch of white sands beach and body boarding able surf.
For the hiker, swimmer, surfer I suggest getting a good hiking book and a book on the beaches to find all the spots the tourists aren't bussed in and still have the tranquility of water dripping with no obtuse chatter.
When the winds are up and the surf is fine, the windsurfer's do the most amazing tricks - full flips, sideways spins and just plain catching air on a wave. Checking them out at Hookipa or behind the Kahului airport is a good free live performance.
Best Way To Get Around:
Maui has no public transportation to the spots you'd probably want to go to. The most common ways to get around are to know someone with a Maui cruiser, buy a Maui cruiser (cheap!) for your trip's duration, rent-a-car (new or used!), hitch-hike, grab a shuttle tour, or plan every day of your trip with an activity where they pick you up from where you are staying.
All though, I'm not sure if it's kosher to recommend, hitch-hiking is a common way for the low budget traveler with no time constraints.
For the low budget short term traveler renting a wreck might be the best. Try Good-Karma Cars.
For the low budget long term traveler buy a truck, van or economy car depending on your travel plans and shelter arrangements.
For the luxury carry me around I have some money traveler, rent a new car and go slow on the rocky roads, take a shuttle or plan each day with a wonderful excursion.