Hāna
You cannot do everything in one day — you will need to choose. Non-negotiable stops: Ke'anae Peninsula (about halfway, lava church + taro fields + Aunty Sandy's banana bread — order extra, it sells out), Waianapanapa Black Sand Beach (reserve separately $25/person), Hamoa Beach after Hana village, and Pipiwai Trail at Kipahulu for the bamboo forest and Waimoku Falls. Skip the Hana Lava Tube if time is short. Fill gas in Paia before departure — the only station in Hana can be closed. The back road from Kipahulu back to Kihei (~30 min of unpaved road past the Pools of 'Ohe'o) is prohibited under most rental car contracts and not insured; double back the same way.
Purchase the Shaka Guide app ($3) before leaving — GPS audio commentary at every stop is essential; without it you will miss the best pull-offs with no idea what you are looking at. Depart by 7am from Paia, 7:30am from Kihei. Fill gas in Paia. Waianapanapa Black Sand Beach requires a separate $25/person reservation at recreation.gov (book 2–4 weeks ahead). Do not attempt as a day trip with children under 8 — it is 10+ hours.
Waterfalls are fuller after rain. Drive slowly — the road is safe but narrow. Do not attempt if fatigued.