If you're coming to Maui on a cruise ship for the love of God rent a car! You can't come all the way here and just hang out at the somewhat pathetic strip mall across the street from Kahului Harbor. Kahului is not a tourist destination. It's where we go to shop, get things repaired, and go to the airport. It's not a place to spend a day on Maui while on vacation.
A friend of mine arrived by cruise ship and had 2 days on Maui but I only got 1 with her so, this is what we did:
From Kahului Harbor, we went to Iao Valley (pronounced E-ow. Ow like I hurt myself.) Advanced reservations are now required: https://gostateparks.hawaii.gov/iao-valley Iao is located about 15 minutes from the harbor and maybe 25 minutes from the airport if you're flying in for the day.
Iao Valley is like a Jurassic Park area. It's a good photo stop. It just feels good to be here. The main walking pathways are short and in a compact area. You can spend as little as 30 minutes here if you just stay on the pathway. For more adventure you can follow the stream down into the valley and jump in. There are also some more advanced hiking trails to explore. Be mindful that Iao Valley is also a culturally significant and spiritual place for native Hawaiians.


From Iao Valley, we drove south to Kihei to snorkel. Kihei is about 30 minutes away. If you're going to snorkel or go to the beach it's best to do it early as the wind generally picks up in the afternoons. You can see my snorkeling tips here. On this particular day, it was way too rough to be snorkeling but we did it anyway as it was her only day to do it. Pretty nerve wracking for a beginner. But she did see a turtle so I guess it was worth it. It was stressful for me though trying to keep track of her in the rough surf and make sure she was ok.
Next, we drove about 15 minutes further south to Makena State Park aka Big Beach. They now charge a parking fee and an entrance fee. Not a fan. https://dlnr.hawaii.gov/dsp/parks/maui/makena-state-park/ Many visitors just park on the side of the street outside the parking lot because they are just coming to take a photo. If the sun is out, it's one of the most picturesque beaches on Maui. It's nearly a mile long of deep white sand. The island of Kaho'olawe sits in the distance as well as the backside of Molokini crater.
Be careful if you decide to get in the ocean here. It's one of the most dangerous beaches in the entire state of Hawaii. It's known for having dangerous shorebreak. More often than not, if your feet are touching the ground - you're in the danger zone. Go under the waves don't try to jump over them. I've seen many people get pounded here.
There are no showers or bathrooms here. Only port a potties in the parking lot. Stop at Makena Landing on the way back to Kihei for showers and bathrooms - if you can find a place to park! There are facilities both to the left across the street from Keawalaʻi Church or if you turn right you'll see a narrow parking lot.

After Big Beach we continued further south to the end of the road, La Perouse Bay. I rarely venture all the way down there. Honestly we could have skipped this stop but we were close enough so might as well. This is some people's favorite place to visit so consider it. It's a massive lava field with no cell phone service. People come here to hike, fish, surf, stargaze, and flip off Jeff Bezos...maybe that's just me. That's his house down there. We just took a few photos and left. Smallish parking lot and only port a potties here.
An alternative stop not as far down is Ahi'i-kinau Natural Reserve. This area is somewhat similar as it is surrounded by lava. The parking lot here is big and paved. Conditions permitting it's a great place to snorkel. There's no sand. You have to trek through the lava field to get there. However this is another place you have to pay to goddamn park. It didn't used to be like this! There are no showers or bathrooms here either just a port a pottie in the parking lot.
Makena is stunning but lacks any convivences. It's a wild place and I like it that way.
Then we got lunch. Aside from the food trucks parked along the road in Makena, which are great, if there there, the first place you'll come to after heading back north is the Shops at Wailea. This is a high end mall. They have restaurants like Ruth's Chris Steak House and Tommy Bahama.
If you're still wet from swimming then Waikiki Brewing Co is really the only more casual place here (this restaurant has changed over many times over the years so double check to see what it is now). Island Gourmet Market located in the north parking lot has coffee and a lot of prepackaged things ready to go if you want to grab something quick and keep exploring. They also have a hot food counter.
Surrounding the Shops of Wailea on both sides are luxury resorts where you can have an outrageously priced meal.
If you travel up Wailea Ike Dr toward the highway from the Shops there is Manoli's Pizza and a few other newish places I haven't tried yet. I like Manoli's. It's an open air restaurant on the golf course not too fancy.
If you want to keep it causal head into Kihei town. Just a few options to consider: Maui Brewing Co (plenty of parking/huge facility); Moose McGillycuddy's (tiny parking lot but across from the ocean), Monsoon Bar & Grill (Indian Food on the water); Coconuts Fish Cafe is a causal counter style with plenty of parking they have two locations; Da Kitchen for local food but you have to take it to go because there's only 3 tables inside and it's always busy; Three's Bar & Grill; The Food Truck Oasis; South Maui Fish Co if you can find it. It's a tiny food truck inside a small, hot, and loud courtyard across the street from the fire station. There's always a long line and there's almost no parking, few places to sit in the aforementioned loud hot courtyard - but the fish tacos are my favorite! You get the most fish for your buck here. Oh, and they keep island hours so who knows if it will be open.
Anyway this first half of the day was a lot! We went to another beach in Kihei after lunch. See below for some alternate ideas.
Before sunset I took her to Ho'okipa on the north shore in Paia to see the sea turtles and then took her back to the ship. Exhausting.

Here are some alternative ideas:
Include a stop at the Maui Ocean Center in Ma'alaea. Consider this dry snorkeling. They have a shark tunnel and a very cool short 3D film on humpback whales in a domed theater. You need about 2 hours here.
This would have been a good place for us to stop after lunch instead of going back to the beach. We could have had lunch here too.

The morning snorkel cruises out of Ma'alaea harbor to Molokini all depart very early in the morning and are 4-6 hours long. It's about 20 minutes from Kahului harbor. If you can't make a morning trip in time, you can take a shorter afternoon snorkel which usually stops at Coral Gardens. Or consider a sunset dinner cruise if you don't care about snorkeling.
If it's winter - go on a whale watch. These typically go out every 2 hours. But if it is whale season I recommend a snorkel trip over the standard whale watch because then you get the best of both. You still get to see whales and you get to snorkel. You'll probably hear the whales under the water too.

Not including the Carl's Jr at the gas station and the new Starbucks in the parking lot, there are currently three restaurants in Ma'alaea at the moment. I love Point Break Pizza & Panini.
You have to pay to park in Ma'alaea now too. Boo!
Another option would be to check out Paia town on the north shore. Paia is home to maybe the most famous restaurant in all of Hawaii - Mama's Fish House. If you plan way ahead then maybe you can get a reservation there. Personally I'd prefer going to the Flatbread Company. If you haven't noticed, I'm not fancy.
You could stop at Baldwin beach which is the Big Beach of the north. If you're feeling ambitions you could continue on further to Twin Falls in Haiku to hike to a waterfall. You gotta pay to park at Twin Falls. They are closed to tourists the first Saturday of the month.
On the way back from Twin falls towards the ship, you could stop at Ho'okipa beach to check for sea turtles on the beach and watch the surfers.
Another option after lunch, or if you're not a beach person consider going upcountry to Kula, some stops up there include: Kula Botanical Gardens, Kula Country Farm, Ali'i Lavender Farm, Alpaca farm, Ocean Vodka Distillery, Surfing Goat Dairy. The views from up here are killer and I truly enjoy visiting all of these delightful farms. Plus it's much cooler temperature wise up here.
Haleakala National Park is also up this way about another hour past the main Kula town. The colors inside the crater are other worldly. It's like visiting another planet up there. Much much cooler as it's above the clouds. The elevation is 10,000 feet.
If you wanted to use your whole day you could drive the infamous Road to Hana. That's not high on my recommendation list. Being in the car all day with 617 hairpin turns, 50 one-lane bridges, only room for maybe 3 cars per waterfall, the occasional mudslide, construction, or downed tree it's a motherfucker of a day. I haven't liked being a passenger or the driver on this trip but to each their own. People actually do this for fun! Waiʻānapanapa State Park is gorgeous. You need reservations. Red sand beach can be a little bit treacherous to get to if you can find the entrance and a place to park. Be warned there are naked people here. The horror!
Finally, you may have noticed I haven't included anything on the west side of the island (Olowalu, Launiupoko, Lahaina, Ka'anapali, Napili, Kapalua). While the businesses that are over there would love to have you. I haven't been able to bring myself to drive west of Ma'alaea since the fires. We are approaching the 3rd anniversary and it's still just too devastating and that's all I can say.