I would strongly recommend a car for your visit. I stayed there for 17 days once and without a rental car, you would be pretty limited unless your doing private tours, local bus or uber.
Rental cars tend to be pricey but there are some local mom & pop rental companies. You would want to have a rental car for Road to Hana.
I would also recommend getting an Air BNB or equivalent. The hotels are expensive and tend to be in two areas: Waliea and Kaanapali. Both areas are great but you may want to pick a location that is central to the two hotel areas along the coast if you're doing an Air BNB. Kihei is always littered with rental units but their beaches are decent. Thankfully other beaches are very close. If you're looking for cheap rentals, check listings for AIR BNB or Tripadvisor with "Maui Vista" in the property name. Its an older building and about a 2 minute walk to the beach.
If you have to pick between the two hotel areas, base it around what you want to get out of it. Kaanapali has more shopping options and is closer to Lahaina (Tourist town). The water conditions and colour are also drastically different between the two.
If you're looking for waves: Waliea. If you're looking for calm gentle water: Kannapali.
Keep in mind food; whether made for you or bought from a grocer is hella expensive. Do yourself a huge favour and sign up for a Costco card before you go. The Costco is close to the airport. This really only applies to you if you have a rental unit or a hotel room with a kitchenette.
Gas pricing is not too bad. I went through about 2.5 tanks in about 16 days.
Some must do's:
Road to Hana (would recommend rental car)
Sunrise at Haleakala (Book parking spot in advance)
Old Lahaina Luau (Book well in advance)
Ulalena Show in Lahaina
Big Beach and Little Beach (Drumcircles on Sundays at Little Beach, its a nude beach though, devils lettuce everywhere)
Kannapali resort Beach.
Surfing (if wanting to learn: Maui Wave Riders in Kihei)
The interior stuff on the island is meh.. unless you like horseback riding and farm tours)
if you're looking to not rent a car and stay in a resort, I would strongly recommend Oahu, Wakiki or Honolulu areas.
if you have any questions, just ask