The first time we went to Kauai we toured the entire island on an all day trip. Probably the main stop was Waimea Canyon, the Grand canyon of Hawaii.  The whole island is gorgeous.

The second time was a shorter excursion up a local river to a grotto area that is used for weddings a lot. We then went up into the mountains for an overlook view of the river.   It was really pretty and a fun time.