We added 3 days on Maui to our trip this past summer. I have a video my daughter made of Aulani & Maui. It is a few post down. It shows alot of what we did.
I want to see both Kauai and Maui again, but I think it'd be a lot easier to spend a week on Maui without repeating much. Road to Hana (day or, better, two), beach day, Haleakala, beach day, Molokini, beach day, Lahaina, beach day.......see a pattern??!?!