If you are planning 20 days that should be plenty of time to visit Hawaii – most people go for a week or less (but to one island). I’ve only been to 3 of the main islands so far – Maui and the big island were my favorite spots. I love the road to Hana and the area around Hana on Maui – its not that crowded and you can easily escape into nature.
There are a number of articles about Hawaii on my Journals page – just type in Hawaii in the search box on the left side and they will come up.
With 20 days, perhaps even consider a trip 4.5 hours more to California 🙂 (esp since you will already have your USA visas).
~ Dave