Virtually all the Luau's have a service (relatively inexpensive) that will come get you and bring you back to your hotel.
If you are spending a few extra days in Oahu, you may want to look into a different experience altogether. Polynesian Cultural Center. Located on the Northeast shore. Spend the day (opens at noon) then do their Luau for dinner and then go to their spectacular show afterwards. You can purchase a complete package that covers everything including admissions, Luau, show, souvenirs, special seating at the nighttime show , guided tour in a vey small group during the day and more. A very rewarding and entertaining day.