While Maui is easily navigable with a rental car, be aware of traffic on the Road to Hana and high parking costs in some locations. Consider renting a bike or scooter in some locations. Taxi's and ride-sharing services are readily available, however, it can quickly add up. Consider booking activities through companies that include transportation.
If you have more time: consider visiting the Iao Valley, the Maui Ocean Center, or the Alii Kula Lavender Farm. If you have less time: combine some activities into one day, such as a morning at Haleakala National Park followed by an afternoon in Lahaina Town. Don't forget, Maui has beautiful beaches all around, so plan some beach time into your itinerary regardless of your time.