Oahu/Kaui Restuarants
If you want to experience great food on your 20 day trip I would recommend:
Oahu
Alan Wongs (absolutely the best)
Roy's Waikiki
The House without a Key
Hula Grill and Dukes are fun....good food. The top 3 are really terrific.
Kauai
Roy's Poipu
Red Salt in Poipu (new and excellent)
Donderos at Grand Hyatt Poipu