You can do that, if you take early and late flights, but, you will not be able to see everything, and what you do see, will be a quick 'look at that, now lets move on' kind of trip. So, if you go into it with that mindset, you will get a lot out of it. Traveling without luggage is so much easier. There are plenty of shops in Oahu, so don't bother souvenir shopping or saving money buying food in bulk at Cosco, just eat a Starbucks or restaurant. Might cost a few bucks more, but worth it to save time and enjoy the island life. I did that on both Maui and Kauai with my Dad who took a cruise there to see what my wife and I love about Hawaii and I met him on those two islands so show him around.
In Kauai, we left Lihue and drove to Waimea Canyon to the far lookout and a couple side roads, then headed back down to Poipu and then the back way on Old Hulemalu Road to the Airport. We arrived around 4. Yes, you might be able to add a couple hours with later flights, but not enough to go up to Hanalei in the same day. It is also very dark and you will not see anything after dark. Traffic jams can add an hour to just travel a couple miles at some common choke points depending on the time or day, so be careful. The top of the canyon can also be shrouded in clouds, with only a one day trip, if it is clouded in, you will see nothing.
In Maui, we left Lahaina at 8 and drove around the West Maui Mountains with stops in Iao Needle, a couple overlooks on the north shore and walked the beach walk in Kaanapali, as it was whale season. We finished at 4. Again, you could add some extra stops, and you can do the same route from Kahului, but adding Haleakala or the Road to Hana would not work. Haleakala is about 1.5-2.5 hrs each way, depending on stops and traffic and clouds. The top is often cloud free, but you may have to drive through clouds hugging the mountain half way up to get there. Road to Hana is a long long day by itself. The road along the north side of the West Maui mountains is drivable, but not for the faint of heart. Many one way blind corners and narrow cliff edge drives, but worth it if you can do it. Do not attempt in a storm.
Of course there are many side trips and stops you can make, Leodas or Star Noodle on Maui are two of our favorite places to eat. There may be a line for Leodas and they may be out of pies if you are later in the day. Star Noodle will most likely need a reservation weeks in advance. There are hikes and waterfalls everywhere and the best art gallery on the north side of West Maui. Can't remember the name, but you can't miss it as it is a huge mansion like house in the middle of the most amazing scenery anywhere. In Kauai, there are also hikes and waterfalls everywhere as well as Spouting Horn in Poipu and some Caves just east of Poipu. There is a long coastal sidewalk along about 1/3 of the island by Kapaa. You might also be able to find a white pineapple ( Sugarloaf ) while in Kauai. Less 'bite' than a regular pineapple, sweeter, less fibrous and oh so good. Keokis restaurant in Poipu looks like a grass shack/tiki bar. They have live music around 5:30 every night. In the middle of Poipu, but you wouldn't know you were next to a road and stores. No walls, so you are surrounded by trees, a pond and trade winds as you eat. Might even spot a gecko in the cracks in the wood posts if you look close.
We have been to both islands many many times and have never had more than 10-15 minutes waiting to get a rental car from either airport. In Maui, it is a 2-3 min walk or trolly ride from the front curb of the airport and in Lihue, it is a 5 min shuttle ride from the airport. Of course, we are always fast to get off the plane. If you are slower and the other 200 people from the plane get there first, you could have a wait since there are not that many attendants at the rental counters.
If the weather is nice, it will be an amazing trip and so worth it. The islands are in many ways the same, yet also very different from each other. Our daughter lives in Oahu, and after being in the other islands, Oahu feels like being in Denver with an ocean ( huge crowded city with skyscrapers, smells, tourists and crazy traffic ).
In Kauai, it is easy to feel you are on a deserted island. Skies are so dark at night, you can find many areas where you are the only person around. It has areas that are desert and areas that are jungle. You can even drive down a dirt road with a red and black rock mini canyon in a pine forest that you would swear was Colorado if you didn't see the ocean. Other areas, just around the bend, make you think you are walking with Gilligan looking for rescuers. The air, oh how sweet the air smells. Unreal.
Maui is magical. Seeing the other islands just a few miles away is travel ad material. There is so much more to do, and a lot more people, but you don't ever feel crowded like in Oahu ( except when stuck in traffic ). Driving up to Haleakala and watching planes flying into Kahului BELOW you is a trip. IF you go to Hana, you can try Hang Gliding Maui. My daughter flew with Armin and he is amazing.
