I haven't determined whether you will have access to a car on Oahu. IF you do, you can just about pay for the nightly parking fee at your hotel by driving to some (relatively) inexpensive eateries (and especially take-outs).
I would agree that it makes no sense to spend all your time driving around looking for bargains. Having said that, IF you can make it part of the Hawaii "adventure" you CAN save a bundle and still eat reasonably well AND healthy.
Some suggestions:
Slightly "institutional" in taste, but we really like to stop at ANY Times grocery store. Go to the deli. They offer 3 or 4 plate lunches/day (take out in a styro box) for about $8 on average. The menu varies location to location and day to day, but they have items like salmon, sliced steak, sliced pork, garlic shrimp, meat loaf, hamburger steak, and others. With entree you get 3 choices of mashed potatoes, fried potato choice, mixed veggies, salad AND a large fountain drink. WE can get TWO meals out of one styro box. WE like the taste, though you'll want to trim a bit of fat and gristle, etc., occasionally. WE take our boxes home OR to a nearby park. They may have a very limited outside seating area, but I'd opt for a park.
L and L (google locations). Decent food (if you like it) and lots of it for maybe $8 - they're everywhere. Very "local" food, maybe not the healthiest, but usually offer a decent fish. Lots of starches available. You'll be full, but will not rave about the food.
Yummy's BBQ (several locations - like Ala Moana food court.) $10 or so for a meal. Very good if you like this type of food.
Heh, heh, Panda Express. Also everywhere. We won't eat at Panda on the mainland (hauole's can't cook Chinese food IMHO). BUT on Oahu, for $10 you can stuff yourself with excellent wok'd veggies and or fried rice or chow fun plus at least 2 very tasty (not all healthy) entrees (Beijing Beef, Orange chicken, beef and broccoli, etc. etc.) Add a refillable drink for $2. Split 4 entrees and one drink between two people and you'll waddle out for $7/head.
Mentioned Ala Moana food court. 20+ choices at maybe $12 average/person - bring your own canned sodas. With several people, try splitting meals at 2 or more places. Pizza ('nyeaaah!'), decent burgers, decent Thai, Japanese, Vietnamese, Panda, etc. You can share your way to less than $10/head. Oh, and 3rd or 4th floor of Ala Moana, there are several sit-down places (Mai Tai, Bubba Gump, Tanaka's of Tokyo, etc.) Not cheap, but not outrageous for lunch - plus decent atmosphere and even "water views" at some restaurants. You COULD walk to Ala Moana from the Ewa end of Waikiki.
Cinnamon's at the Ewa end of Waikiki. Sort of an upscale (no, really!) Denny's. $1/hour parking behind. Same area there is a Dead Lobster (oops! I mean Red Lobster.) Some other non-chain restaurants in the area you could browse - especially for lunch.
Speaking of Dennys - at the far Diamond Head end of Waikiki - on Kapahulu - across from the zoo - is a Dennys. Not our favorite Denny's on the Island, but with AARP discount, DW and I WADDLE out for $10 total WITH TIP (okay, maybe $12 total.) We do the $2-$4-$8 menu and drink water.
On Kalakaua ave a couple of streets Ewa of Kapahulu is Wolfgang Pucks. It ain't local. We split a nice salad and one entree for LESS than $18 or $20 TOTAL WITH a refillable drink and a chunk of banana bread or similar. We "tourist watch" from the outside tables. The beach is across the street.
Costco "window" has same fare and prices as mainland. Best pizza on the Island - sad to say. $2 for meal-sized slice. $1.50 for huge dog and refillable drink. Enjoy!
You don't have to pay a fortune to eat - even in Waikiki area. You just have to be flexible in your tastes and maybe be able to get a mile or 2 outside of Waikiki.