I love Hawaii! I'm on the fence regarding your lodging. At around $353 per night, I don't necessarily think that's a bad rate, depending on where you are staying, how many kids you are taking, and that you are visiting in December. I'm assuming 2 kids (at least when we visited said max 4 people in a room). My suggestion, and we've not done this before, is to go to VRBO.com and type in Hawaii Waikiki just to get an idea of prices. You may be able to get your lodging much cheaper--don't know, as Waikiki is expensive, but so worth it!! I've not used VRBO before, but may the next time we go. You might be able to find a rental with a kitchen, so you can do some of your cooking while you are there. But being that you are travelling in December, most places are probably already booked up.
You could get a membership at Costco and then cancel the membership, but Costco tends to sell stuff in large/bulk quantity, so may not be worth it.
Don't know where you are staying, but Hilton Hawaiian Village (that's where we stayed) has a Friday night fireworks show (every Friday)----it's free---just ask your hotel where to go to see it. It's by their lagoon.
There a International House of Pancakes (IHOP) by HHV that serves good breakfasts that are reasonably priced. They are quite busy, but worth it to go. And you could walk there (depending on where you are staying on Waikiki).
Some hotels charge a daily resort fee, or a parking fee. Could you provide more information regarding this? I'm not sure you can get around these fees. It's the price you pay for staying in paradise. :)