Have you looked into the Carnival Spirit on Sept 18th?
It's $60 cheaper than the lowest NCl sailing, 5 days longer, and still visits Hawaii for 7 days. Also you don't have to worry about the poor reviews NCL seems to have here. Check on the flight as well, for me doing a multicity through Vancouver was cheaper than RT Honolulu.
What kind of activities do you do on ship? Any particular sites that are must see in Hawaii? I've never tried but would love to do some surfing lessons