Activities (zipline! Snorkel trip! Parasail! More!) can easily cost over $100 per person per activity. Meals are KILLER. Get a nice condo (with a pool in the complex), go shopping at Costco and the local markets & DIY all the meals you can. HUGE savings, especially on Maui. PB&Js are your friend.
Do you like pineapple? Do you have a Costco membership? Buy pineapples at Costco! (NOT the green ones ok? You buy the Maui Gold pineapple that is the orangest/yellowest you can find, and if you're not sure the color is right you should be able to smell that wonderful pineapple smell.) Last time I was on Kauai we bought them at Costco a couple days and then when there was a farmers market nearby one day all the good ones were gone. We went to the farmers market to look for one and they were selling them at twice the price of Costco and I literally saw them removing Costco tags before setting them out to sell... Also, Costco is where we pick up all our Macadamia nut chocolate/coffee/various flavored covered souvenirs! As well as the replacement set because we basically eat them all before we leave. And gas for rental car!
Merchandise: Kmart & Long's Drugs. Most of the trip souvenirs you want can be found there much cheaper than tourist shops & hotel gift shops. Actually all our Hawaii t-shirts came from Long's last trip! (Yep, it's still a thing there.) I majorly splurged on a Hilo Hattie dress at Hilo Hattie though last time I was on Maui so don't think you'll be getting off cheap on everything, you never know what someone might fall in love with
FWIW you could easily spend a week on Maui and stay occupied without having to hop to Oahu.
Oh! And rent snorkel gear for the week so you can do some of your own snorkeling! It's not Molokini Crater but still pretty in the right places to snorkel if you're looking to occupy an hour or two some days. (Avoid discounted afternoon snorkel trips, odds are the water will be rough so you won't see as many fish.)