I went with DD and my parents last August. We stayed on Oahu the entire time because I had to be there for work for 2 of the days. The main thing my dad wanted to do was visit Pearl Harbor and my mom is a nervous flyer. All that together made it seem most reasonable to stay on just the one island. If I were to go back, I would want to visit the Big Island and Kauai.
I started looking for airfares around mid-January and around the last couple days of January, the fares dipped down from around $1000/ticket to $700/ticket. Still extremely pricey, but a decent savings.
I found the best car rental rates at DiscountHawaiiCarRental.com. You can cancel anytime without penalty and if rates improve, you can cancel and rebook at the lower rate.
We stayed at traditional hotels, but there were much better rental rates at condos through vrbo.com. A bonus with a condo is that you could cook some meals in rather than eat out all the time.
I was really surprised to find reasonable prices for restaurant meals. The trick, at least on Oahu, is to search out smaller, mom and pop type restaurants. The restaurants in the main tourist areas and in hotels were not cheap, but there were plenty of better priced options. Tripadvisor.com was essential for locating some of those places.