The last time we did Road to Hana someone recommended renting a Jeep with an open top for the day. It was totally worth the extra cost and made the trip extremely enjoyable. The two spouses with us who had struggled with carsickness in prior trips rarely complained, and loved having the open air and breeze. Of course adaptations are necessary for bad weather... but we lucked out.

I also recommend starting early and avoiding the crowd later in the day...