You will find river cruising amazing! We are definitely hooked on them and can't wait for the next one. They are entirely different than ocean cruises in every way. And there are so many wonderful itineraries out there to choose from, not to mention many wonderful river cruise companies.
As for Hawaii, we spent 3 weeks there last year, including a one-week cruise on the Pride of America. Truly awesome cruise! While the ship is okay, the crew was fantastic. The food wasn't all that good, but we didn't do the cruise for the ship - we did it for the itinerary and we were not disappointed. Great way to see the islands. One suggestion; don't need to do any excursions. Best to just pre-reserve a rental car in each port and do your own thing. Definitely cheaper, easier, and affords you an opportunity to do what you want, go where you want, and see what you want without the high price and crowds.