Hawaii is expensive as far as airfare, lodging and food goes, but there are tons of cheap activities that help you keep a cost of a trip down.
DD and I just got back from 12 days on the North Shore of Oahu. (I don't like the crowds of Honolulu but love the North Shore.) We rented a 1 bedroom beachfront condo on VRBO in a small complex we've stayed at several time before (DD slept on the futon in the living room). We went out to eat some, but cooked in probably 70% of our meals - it's not so hard to cook when you can see the ocean from the kitchen
(I pack things like spices with me, but buy all fresh and frozen items at a local supermarket once we get there.) We had a rental car the entire time - we like to get out and roam on the island and can't imagine not having one. For activities we hit the Pacific Aviation Museum (we had previously done the rest of Pearl Harbor) and a couple of smaller museums, did some hikes, etc. We've been several times before, so we've seen many of the standard sites before and so we tend to hit smaller sites now. We also just spent lots of time on the beach right in front of our condo, snorkeling, swimming and spotting turtles!
Total cost of our trip including everything (incl. things like boarding our cat, Uber to/from the airport, etc.) came in just under $4500.
One thing to look at to reduce costs is the Alaska Air credit card - once you hit the minimum spend, you get a companion pass that allows you to get a second ticket for around $150. That can be a huge cost savings. It also gives you and a companion on the same reservation one free checked bag each. That saves even more.
For rental cars, check all the standard websites for the best rate, plus discounthawaiicarrental.com. Book a refundable reservation for the best rate you can find when you first book your trip, but then keep checking constantly for cheaper cars (at least weekly if you are more than 6 weeks out, then daily if you can remember). My car rental started out at about $500, but it then started dropping about 3 months out and l finally got the best price of under $300 about 10 days out from our trip. I could have gotten it even slightly cheaper that that, but I wanted Hertz specifically because many of the car agencies in Honolulu are offsite, but Hertz is onsite. The time savings picking up and returning the car in this case is dramatic so I'm willing to pay a few more bucks to go with Hertz. Note Hawaii is also a unique rental car market in that compact cars often cost more than the bigger cars. That's because the compact cars (1) don't scream "tourist" quite as much and so don't make you as big a target for theft, (2) are easier to maneuver in tight parking situations, which you will often encounter on the islands and (3) the gas mileage is better, so it is cheaper to really get out and explore whatever island you are on.