Kaua'i is considered the most beautiful of the Hawaiian Islands. The Nepali Coast is an unbelievable site. The western part of the island is home to Waialeale, one of the wettest places on earth, which is connected to Waimea Canyon the driest place on island. The contrast is mind blowing!

The North Shore is equally amazing, and Poipu in the South is a classy, but gorgeous ( see below )

Cycling in Kaua'i is easy since the bus has bike racks for three bicycles in the front. You can take the bus to Poipu ( with the Koloa shuttle), and the north shore ( with the north shore shuttle ). However going to the Waimea Canyon is more difficult. To get to the Canyon you will take the bus to either Waimea or Kekaha ( there are two roads to the top of the Canyon ). It is a 14 mile ride to the top with an elevation change of 1 mile ( approx 7% average incline ). 

I went to Koke ( the park at the top of the mountain ) on many camping trips. I never saw a cyclist on the road up or down. Hence you may want to rent a car to get up to Koke...it will be worth it.