For Maui, you will definitely want to rent a car. The Hyatt is quite a ways from the airport, so by the time you pay for rount trip taxi you may as well rent a car. You will also want to have a car to go to Lahaina at least one night. The Old Lahaina Luau is near the north end of town, but as others have said is too far to walk. Also, make sure you try Aloha Mixed Plate right next door. Very casual place with good Hawaiian comfort food.
Another reason you will want a car is to do at least one drive while you're there, either the road to Hana or the drive up Haleakala. Both are great but I prefer the road to Hana. Make sure you stop at the small stands and sample the local banana bread.
AGS