Among the waterfalls that you pass on the Hana highway, Wailua Falls is perhaps the most impressive depending on weather and water flow. Keep in mind this waterfall is located 7 miles past Hana, in case you wanted to see it but were planning on turning back at Hana.
There's a reasonable amount of parking across the bridge and sometimes roadside vendors selling items. You'll stop for perhaps 10 minutes here to take photos. It's possible to walk to the base of the waterfall but the path is slippery and difficult.