The Road to Hana hugs tons of brilliant, jagged black lava shore. Around mile 16, pull of the road to explore the Keanae Peninsula. Thee're an old Hawaiian village, taro patches, and a church that dates back to the 1850s. I love watching the waves crash against the lava pinacles - the contrast of the jet black stone against the turquoise water is just brilliant.
Keanae is also home to Aunt Sandy's, one of the best banana bread stands along the Hana Highway (you'll see them about every half mile). Make sure you pick up a slice for the rest of the ride.