My family did Big Island/Maui in 10 days just a few years ago.
Personally, I’d stay 3 nights on Big Island, then spend the rest of the time on Maui.
Here’s what we did:
Flew into Kona airport from the mainland. Rented a car with National because it allowed pick-up in Kona and drop off in Hilo, which are on opposite sides of the island.
After arriving, we ate dinner then hit the sheets at the Marriott Waikaloa. You’ll be tired after that long plane ride. The next morning, we hit a few beaches/ snorkel spots, beginning with 2 Steps.
For dinner, we ate a local restaurant and chatted it up with some locals. About a half hour before dusk, we met our captain at the pier for a night manta ray snorkel. (If you SCUBA, they offer dives, too). You take a boat ride and watch the sunset. Then you get to a spot where someone has dropped several light rings into about 20-30 feet of water. The light attracts plankton, which attracts these huge manta rays. You get in the water with them and watch them glide within feet of you. It was magnificent and I highly recommend it, even though I got pretty sea sick. (Take your Bonine if you suffer from motion sickness.) We spent the night at the Marriott again.
The next day we checked out and headed to Hilo. If you go clockwise around the island, you can stop and Zipline on your way. It took us about 5 hours to get to the other side of the island, including stops for lunch and sightseeing. I think you can get there in about 3 hours if you don’t stop.
We arrived in Hilo at dusk. We’d booked a room at Aloha Junction B&B, which is about 5 minutes outside of Volcanoes National Park (VNP). Ate dinner at Kilauea Lodge (make reservations ahead of time). (You can also stay at the lodge or inside VNP at Volcano House, but we liked Aloha Junction and it was very reasonably price.) Then we headed out to see the lava at night. It’s one of the few places on earth where the ground you’re walking on is younger than you.
Spent the entire next morning and early afternoon at VNP, hiking the trails and the cave.
We booked an early evening flight out of Hilo to Maui on Hawaiian Air. Renting a car with pick-up on one side of the island and returning it on the other side of the island saved us several hours of driving. I’d highly recommend this. (Note that mainland flights fly into Kona, so plan your trip to arrive from the mainland into Kona (if you do BI first) or depart from Kona (if you do BI last)).
I’ve been to Maui a half dozen times now. It’s my favorite island by far. We usually stay in Ka’anapali, but have also stayed in Wailea. We prefer West Maui because it’s closer to Lahaina than South Maui. Lahaina has tons of shops and restaurants and it’s also one of the only 3 deep water harbors on the island, so if you plan to spent lot of the water, it’s very convenient.
What to do on Maui? Depends on what you like to do. My top 11 things;
Trilogy’s All Day tour to Lanai is my #1 activity. It’s pricey but awesome. You should see plenty of whales in March on your way over.
Snorkel at Honolua Bay (ocean conditions permitting). If conditions are rough, park and watch the surfers tear it up.
Spending the morning at Ho’okipa Beach and watching the surfers and wind surfers, then lunch at Mama’s Fish House. Or pack a lunch from one of the restaurants in nearby Paia. Visit some funky shops in Paia if you have time.
Kayaking trip near Makena Beach aka “Turtle Town.” Go with a guide for safety’s sake.
Morning ziplining on Haleakala with Skyline EcoAdventures (check Groupon or Living Social for a discount), then head to the summit. On your way down, stop by the Visitor’s Center just inside the park entrance and have a picnic lunch. Or drive a short way to Iao Needle State Park, and take one of the trails down to the stream and have lunch on the boulders. (Bring your bug spray). Then hike up to Iao Needle.
Warren & Annabelle’s Magic Show in Lahaina - good, corny fun. You don’t like magic? You will after seeing this show.
Resort day - Nothing better than sitting by a beautiful pool, reading a book, and ordering cocktails from the poolside waiter.
Snorkeling at Molokini - If it’s a clear day, you can see forever. It’s touristy, but so worth it if the conditions are good. Book a smaller boat that arrives earlier than the bigger boats, such as Captain Steve’s. Or, if money is tight, book with Pacific Whale Foundation as they offer the most reasonable tours in town.
Sunset catamaran or sailboat ride - We used to do the Kiele V but it tragically sank a few years ago. Now I’d book with either Trilogy or The America II. Enjoy some cocktails while watching the sun set. Could there be anything more romantic?
Hiking West Maui mountains - Don’t venture off the beaten path if you’re not an expert hiker. West Maui Mountains are the #1 location where tourists need assistance because they got lost. We booked an all day tour that included a hike to a waterfall, then a drive counter clockwise around West Maui mountains (which is an adventure all on its own), to an art gallery, then to Nakelele Blowhole before ending up at DT Fleming Beach for a few hours. Great day.
Bluewater rafting - okay, this isn’t a trip for people with back issues because it’s on a zodiak and the captain FLIES. But we did a really cool cave tour followed by a snorkel trip on the backside of Molokini (very rare to do this on other tours). It was a lot of fun but not for the squeamish.
Things I haven’t done but are on my bucket list:
Helicopter tour - my motion sickness is preventing me from booking this, but I think it’d be a gas.
ATV tour - I love ATVs. Just haven’t had time.
Stargazing at Haleakala - Booked it once, but the summit was too cloudy and we had to cancel. But this sounds like an incredibly great time. Book later in the week after you’ve adjusted to the time difference a bit.
Stargazing on the water - You can only book this a few times per month when the moon is darkest, but I’ve always wanted to do this. Something about being out on the water at night that intrigues me.
Things I’ve done but wouldn’t necessarily recommend:
Road to Hana - A loooong drive, and I get motion sick, so not my favorite activity. The backside of Hana is actually way more interesting than the waterfalls, IMO, but I’d never do this in one day again. I’d book a night in Hana. And if I only had 6 days on Maui to begin with, I’d pass. If you simply must go, know that there are tons of hairpin turns so the driver will get fatigued from having to concentrate. Pack your longs, plenty of food and drinks, and bring TP cause a lot of the potties don’t have it. Oh, and if you hike to waterfalls, you must know what the weather is on top of Haleakala because the streams you’ll encounter are subject to flash flooding.
Horseback riding - Okay, but not earth shattering.
Sunrise bike ride down Haleakala - The national park shut down bike rides that used to begin at the summit due to the number of deaths on the road (picture tons of switchbacks, sun in your face, and a bike sharing the road with cars), so now bikers have to start outside of the national park. That turned a special thing into a “meh” thing IMO. Sunrise might still be worth seeing, but book it early in the trip when you’re still on mainland time and 3:30am won’t be so daunting.
Old Lahaina Luau - It’s supposed to be the best, but it’s a tourist trap IMO. The dancing is good, but the food itself is meh and the syrupy sweet cocktails are terrible.
Hope this helps. Aloha!