I just returned from my Maui, one of my favorite islands. The warm waters, the sunshine, the waterfalls, the rainforest, and the poke!
The road to Hana is a must-see in Maui, but if you get car sick I recommend taking some dramamine well ahead of time. There are many twists and turns on this windy road. Go the opposite way around, from the west side of the island to the east. It's a little bumpy but less windy and then you're not hitting loads of traffic or driving into the sun at the end. I've done this trip 3 times and by far this is the best route.
There is a winery and a general store on the way where you can stop for snacks, wine tasting, and a bathroom break. Maui Wine opens at 10am if you want to time it so you can get some pineapple wine tastings at the beginning—or just buy some for later. If you forgot snacks or plan on hiking you can grab some snacks or a sandwich at the general store.
The first place you should go is Pools of 'Ohe'o (commonly referred to as the 7 sacred pools). As long as the water isn't too high you can swim in these. At the same location, there is a beautiful 5 mile round trip hike to the 400ft Waimoku Falls from there. On the way you wander through luscious greenery, a bamboo forest, see a massive banyon tree, and finally you get to the falls. If you're outdoorsy, it's well worth the trip.
The next stop I would hit up is the Kaihalulu Red Sand Beach. The sand is literally quite red and the bluffs above the beach are stunning. If you have time you can hang at the beach for a while and get into the ocean. The currents can be strong though so I'd be careful in doing so.
My favorite spot visually is the Waianapanapa Black Sand Beach. The colors of the crystal blue ocean and lush greenery against the black sand are stunning. I could've roamed around for hours, but unfortunatley we didn't have a lot of time to explore. If you have the opportunity I would try and stay mid way somewhere along Hana so you can take your time to see all the nooks and crannys.
We went diving the morning after Hana. I've seen a sea turtles everytime I've gone diving, and even a few times when I've gone snorkeling in Maui. Maui Dive Shop were top notch! They prepped all our supplies for us, as well as a great spread for breakfast and lunch. I'd highly recommend them.
We stayed at the Mariott Wailea. It's a more quiet resort style part of the island, with spectacular snorkeling right off the coastline. Loads of turtles! As most resorts go, it was pricey, but the views and infinity pool were great to lounge at. If you prefer more hustle and bustle in the evening, stay in Lahaina. You can get a condo here blocks from the beach for a really great deal, and there's a lot of nightlife.
If you're going to splurge on a great meal (or many!), make a rezzie at Mama's Fish House. It's crazy expensive but it's the best meal I've ever had on the island! And I've been to Maui four times. To give you an idea it's around $50 a plate, $18 a drink.