Hana is a sleepy hamlet at the end of a long and winding road (to paraphrase George Harrison, who had a getaway pad halfway along). The highway's dozens of narrow bridges and cliff-hanging skinny roads are a right-of-passage for Maui visitors, most of whom do a daylong round trip.
But repeat visitors learn that several days of exploring are to be had at lesser-known waysides in this rainforest wonderland. Some spots are well-marked, often by a convergence of hastily parked rental cars. Other trails, some of the best, take a keen eye to spot. A few of the roads into the jungly state-owned lands require a permit from the East Maui Irrigation District, which controls agricultural water flows. It can seem a bit complicated to newcomers. Maui Trailblazer has a detailed description of the Hana Highway from beginning to end. The takeaway: use one day to explore the road, and a second day to see Hana and the Pools of Oheo (the lower section of Haleakala National Park). And keep your eyes peeled.
BTW: Pull over at turnouts to let the local people pass.