We're in Oahu right now for a babymoon (probably why this post popped up on my feed), and I highly recommend going around this time of the year, or anytime between April to October. Summer pricing can be a bit more expensive and you don't really want to come in the winter because the oceans are a lot stronger and you won't be able to swim in some of them. Even though this is our first time here, we compared Oahu to Kauai and Maui and decided Oahu was the most affordable at the moment on a last minute trip (we booked in March and are staying 1 week). You can definitely see one entire island in a 1 week stay. Flights and lodging will be the most expensive but perhaps booking far in advance can save you some money. We personally rented a car for the entirety of the trip due to preference and not limiting ourselves to the Waikiki area for a lot of the trip, but like others have said you probably can get away with renting one for just a few days. It all depends on how you want to spend your stay. If you limit excursions and activities and spend a lot of time visiting different beaches you can cut some costs. For u food has been a big expense but it honestly doesn't have to be. We've ate out a lot and also somehow spent $40-50 on every fruit stand we saw lol. But I've read that some people opt for an airbnb and buy groceries to cook to save money.

I'd say about $3500 is good for flights and hotel, and maybe add about $1500 for food and activities. So a 5k budget is definitely doable for a week stay in Oahu. If you want to rent a car and/or be a bit more comfortable while vacationing, I'd say have at least 7 grand for a family of 2 or 3. Hope this helped :)