only been to hawaii twice [we were saving it for when we get old], but after the first trip to Kauai, there was no question about where we'd go the second time -- it's much more laid back & untouristy, with great seafood in restaurants & grocery stores; lots of great, uncrowded snorkeling sites; inc4edibe variety of terrain, from desert like canyons to tropical jungles to sandy beaches. 2 nights would not be enough
we do expect we'll visit the bigisland on the next trip, but 1 day in Honolulu was more than enough [recommend seeing Pearl Harbor]
Congrats on your trip. You can fly or cruise to any of the other Islands. There used to be an island shuttle service, but some residents were opposed and last I heard it had been discontinued. Kauai is beautiful -- very lush. Visit the botanical gardens while there. Expect a little longer travel time; there aren't many main roads there. Have fun!