If you prefer not to rent a car from the airport, consider booking a private transfer that will accommodate your party. These can be done in advance and will be big enough for 5 people and your luggage and will save you money over taking Speedi shuttle, as they charge per person.
There is a good luau on the resort property every Tuesday evening, Fia Fia, and it includes fire knife dancing. It is one of the top rated luaus in Hawaii. There is another luau at Paradise Cove, at the other end of the Ko'Olina property.
There are several sunset cruises and from our past experience, none were really memorable. They usually leave from Waianae, but some may sail from the Ko'Olina marina. There are probably better options from Honolulu.
There are a number of options for groceries in Kapolei, about a 20 minute drive. Some will allow you to order and they will deliver to the resort. There is a Costco, Safeway, Foodland, as well as Wal-Mart and Target. There is an Island Market, owned by ABC, in Ko'Olina Station and it has some groceries, as well as prepared foods.
There are two restaurants on the Marriott property. Longboard's is a casual restaurant with generous portions, that is great for lunches, drinks, and limited dinner options. They usually have an entertainer in the evening, so you can enjoy a bit of music while you dine. They also have a takeout window, if you prefer to eat in the villa. There is also Longhi's, an independently owned restaurant that opens for breakfast, lunch, and dinner. If you make an advance reservation and ask for a table in front, you can enjoy the sunset while you dine. There is a marketplace on the property that has a few grocery items, but it sells Starbucks coffees, breakfast pastries, etc., as well as some premade salads, sandwiches, etc.
Our favorite restaurant is Roy's, over at the golf club. It is great for sushi and the freshest seafood. We love to eat out on the patio overlooking the ponds with the swans. The Four Season's and Disney's Aulani have several other good restaurant options. At Ko'Olina station, Monkeypod is a favorite, but there is a Mexican restaurant, Eggs n'Things, etc.
Roy's, Monkeypod, and Mina's at the Four Seasons have the best Happy Hours and they will take advance reservations. Monkeypod has the best Mai Tai's.