First things first. Download this app, buy the bundle for whatever island you're on, and get a rental car that lets you off-road. BTW there will always be a chance of rain on road to Hana. Try to do this on the sunniest day you have on your trip.
A friend told us about Shaka Guide before we went on our adventure and it totally enhanced our trip. We did the Road to Hana AND the Reverse Road to Hana guides on Maui. You'll want to download the guide before you leave your hotel and such. We got to see some fun spots that were not as commercialized, or known about and learned a lot about Maui and it's history.
My recommendation --- do the reverse road to Hana route if you're comfortable doing it, I do not recommend doing this route if there's a high chance of rain on the southern side of Maui that morning.
Get started as early as possible. You'll want to pack some snacks, bathing suits, towels, and cash. AND make a reservation at Wai'napapa State Park. If you do the reverse route I recommend the mid-afternoon slot.
The Pipi'wai trail is a MUST do, it'll take around 2 hours and be the most rigorous thing you do all day. Everything else is a stop-go, decide how long you want to stay kinda thing.
With the Maui bundle you'll also get a trip to Haleakala summit, highly recommend going at sunset and packing warm clothes! Aloha, and enjoy!