Among the waterfalls that you pass on the Hana highway, the 80 ft high Wailua Falls is perhaps the most impressive depending on recent rainfall. Keep in mind this waterfall is located 7 miles past Hana.
There's a reasonable amount of parking across the bridge, and you'll often see roadside vendors selling items at the falls. It is possible to walk to the base of the waterfall although the path can get very slippery after rainfall, and you have to be comfortable climbing over boulders.