Maui

Hana Highway - My travel guide ('Moon Handbooks - Maui' by Robert Nilsen) recommended against biking the Hana Highway because "in terms of safety, it is one of the worst." However, I could see no safety problem with the road. Granted, there are lots of switchbacks, but they tend to greatly slow down traffic. Indeed, cars are generally moving only slightly faster than bike speed. What you will see while riding is dense canopy that mostly blocks out views of the coast; thus, I find claims that the highway is one of the "10 best" to be rather overblown. But nonetheless, it is quite a nice ride. Note: there are no services along the route, and the town of Hana is more of a settlement than a town. If you want to stop along the way to eat, your best bet is one of the roadside stands.

Kahekili Highway - This is a great place to ride in Maui. The far northwest tip of the island offers some stunning views from a winding, cliffside road with very little traffic. I started out from Wailuku and did a loop through Lahaina and up Highway 30 through Waikapu. The only downside was stretches of heavy traffic along the western coast. One option for avoiding the heavy traffic is to not do the loop; rather turn around at Kapalua and head back the way you came.

Haleakala National Park - This ride climbs up a 10,000 foot volcano, though I started out in Kula which gives a ~3000 ft head start. If you are used to climbing the East Bay hills, the ride actually isn't that difficult as the gradient is quite minimal, provided you can acclimate to the reduced oxygen. I can verify that it does get a bit cold at the top, so dress accordingly.

O'ahu

Waikiki-North Shore - I did a ride up the east (windward) coast on the Kamehameha Highway. Once outside of Waikiki, traffic drops significantly and you can enjoy a nice scenic ride up the coast alongside green cliffs. However, the North Shore area can see some heavy traffic (usually at a standstill). If planning to continue beyond Waimea to make a loop back to Honolulu, be aware that there are bike-hostile tunnels to deal with. I took the bus back from Waimea.

Kaua'i

Waimea Canyon - This ride is definitely not for the beginner. It begins down at the coast and climbs up to the Grand Canyon of the Pacific. The route is more rollercoaster than road and some sections have very steep climbs. Be sure to continue on the road all the way to the very end, where it dead-ends into a parking lot with stunning views over the Na Pali coast.

Hawai'i

The "Big" island has much more territory than the other islands, so there are a lot more places to ride.

Hwy 137 (Kapoho-Kalapana Beach Road) - Nice and quiet coastside biking with secluded beaches hidden behind trees. The road dead-ends at Kaimu where the highway was buried under lava.

Hwy 190 (Waimea - Kailua-Kona) - Parallels Hwy 19, but has much less traffic and runs way up in the hills, so you get views of the coast below.

Hwy 11 - This highway wraps around the southern tip of the island. I biked from the "town" of Volacano to a beach near Captain Cook.