K.B.
I grew up in HI and still go back every year to visit family. I assume you are going to the Big Island since you want to see the volcano. Christmas is always really expensive no matter how flexible your dates are, so I would definitely look at the March/April time first.
When we went on our honeymoon on Kauai and the Big Island, a travel agent was able to get us a good price on a combination of hotel/car/airfare that we couldn't get online. Otherwise, B&Bs are usually a good deal, especially if they're not on the beach.
Ways to save $$ include eating "plate lunch" at local places instead of restaurants--servings are huge and cheap and delicious; beaches are always free; get a room with a kitchenette so you can fix breakfasts or lunches on your own. You'll have to pay a premium to swim with dolphins, but it's probably worth it--once in a lifetime opportunity, right?
If you have any specific questions, feel free to email me! Hawaii is a great place!
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