Yes, you'll definitely need a car. On Maui, if you stay at the Marriott, it's about 30 mins from the airport and even further if you choose Kaanapali or anywhere near Lahaina. I'm partial to Wailea because that's where we've always stayed on Maui. Have you looked into the Maui Prince Hotel in Makena near Wailea? My friends stayed there last year and had a great time. It's also close to Molokini Crater which is one of the top spots for snorkeling. There are lots of restaurants, shops and grocery stores in nearby Kihei. We usually stock up at the Safeway on the way to the hotel. If you decide on the Marriott, make sure their renovations are already complete or that you're staying away from a construction area. My ds loves the Sugar Cane Train. It's a 30 min ride both ways and the conductor gives a nice tour. We usually stop at the Maui Ocean Center for an hour or two before riding the Sugar Cane Train. Afterwards, we'll head over to Lahaina to browse the galleries and have a meal.
On Oahu, the Marriott is nice with all the lagoons but far from anything else. I haven't been over there in a really long time but they are building new hotels and condos in Ko'Olina so make sure it doesn't affect your stay. I like the Kahala Hotel for the beach and the dolphins but it's adjacent to a private golf club and residential area and you'll most definitely need a car. Waikiki gets a bad rap but it's really trying to reinvent itself. They've opened the Beachwalk and renovated the Royal Hawaiian Shopping center and both of these places offer lots of dining and shopping options. The Royal Hawaiian is a great hotel - it's the pink one - but I'm not sure if it's still under renovation. If you're a starwood member, try the Moana Surfrider Hotel. It used to be a Sheraton but I think it's been rebranded a Westin. Both of these hotels have a bit of the old Hawaii charm.