Just join their frequent flier program, HawaiianMiles.
Here's their fee schedule for members for checked bags on inter-island flights:
Neighbor Island (USD)
1st checked bag $15
2nd checked bag $20
Restrictions may apply. As a member benefit, HawaiianMiles members will enjoy reduced 1st and 2nd checked baggage fees when flying between the Neighbor Islands, traveling wholly within the State of Hawaii. For example, a ticket from Honolulu (Oahu) to Kahului (Maui) is considered a Neighbor Island ticket, and therefore, Neighbor Island bag fees apply.
Source: "Domestic Baggage Allowance" section on Hawaiian's fee schedule
This will save you $10 on the first checked bag per person on the inter-island flights. However, you'll still have to pay the $25 on the flights to/from the mainland.
Another option (which may be combined with the above): Airline fee credits on premium travel cards.
Assuming you haven't already paid the bag fees, you can sign up for a credit card that has an annual airline fee credit, such as the American Express Platinum Card. Other examples include the Ritz-Carlton Rewards Card and the Citi Prestige card.
Also, Chase is launching the Chase Sapphire Reserved card next week (August 21, 2016) with a $300 annual travel credit. So, if you're considering getting one of these cards, it might be worth waiting until next week to see what the details are for the new Chase card before choosing which card would work best for you.
In the case of the AmEx Platinum Card, the airline fee credit will credit up to $200 per calendar year to your account to cover fees charged to the card from airlines. This specifically includes bag fees, seat selection fees, lounge access or membership fees, and on-board fees that are charged through the airline itself (the Wi-Fi typically doesn't count, since it's usually charged through a third party like GoGo or ViaSat.) While the T&Cs for this benefit state that it does not cover actual ticket purchases, upgrades, or gift card purchases, the reality is that a lot of those get automatically credited, too, with particulars varying a bit by airline and discussed at length on flyertalk. While I don't try to game it by purchasing gift cards and what not, I have had some cheap upgrades I was buying anyway get credited back (both to economy+ and to First on Delta.)
With AmEx, the fee credit will only be paid for one particular airline during a given calendar year. You must select which airline you want for that calendar year on American Express' website before making a charge that you want to use the credit on. Hawaiian Airlines is a participating member and there is a discussion of people who have selected it over on flyertalk.
One interesting thing to note in regards to Hawaiian is that you can use the airline fee credit toward a Premier Club membership, which would both cover 2 check bags per flight as well as allow access to Hawaiian's Premier lounges, priority boarding and security, and various other benefits. For a single trip, though, you might be better off just using the credit directly for the bag fees, as the Premier Club membership costs $299, so it would still cost you $99 extra after the $200 credit.
I can't speak from personal experience on exactly how the fee credits work with the other cards, but I'm sure you can find at-length discussions on flyertalk and/or travel blogs. And, of course, read the T&Cs.
Each of these cards has a high annual fee (around $450 USD in most cases,) but they include lots of valuable travel benefits and might be worth your while if you travel frequently. For example:
Most of them will give you a credit that pays for you to sign up for either TSA PreCheck or Global Entry (which includes PreCheck.)
Each of the above cards also gives you access to airport lounges in
the Priority Pass network.
AmEx Platinum also gives you access to the American Express Centurion Lounges and, when flying Delta, to Delta's SkyClubs.
They give you automatic elite status with various hotel and car rental chains (which can end up saving you money and/or earning you more rewards points with those chains if you use them, as well as potentially getting some upgrades.)
Most of them provide various travel and car rental insurance plans.
Each provides a rewards programs per dollar spent on the card.
Each has a sign-up bonus of points that can be transferred to various airlines (except the Ritz card, which has a sign-up bonus of 3 free nights at one of their properties.)
Too many more to list here, but you get the idea.
Note that, due to the high annual fees, I would certainly not recommend getting any of these cards just for the bag fees (obviously, you'd end up paying more than you would on the bags.) However, if you travel frequently enough that the combination of that with all of the other benefits of one of those cards is worth the annual fee to you, then it's certainly an option you can look in to.
Also note that this answer is somewhat USA-specific. I'm assuming from OP saying he's flying from the "mainland" to Hawaii on Hawaiian Airlines that he's in the USA. While some (or maybe all) of these cards are offered outside the USA, the particular benefits offered for each card vary somewhat by country of residence of the cardholder and may not match what I've listed here for countries other than the USA. Be sure to check the benefits terms and conditions that apply to your particular country before signing up for one of these cards (or any other card, for that matter.)
By the way, no, I don't work for or have any affiliation with any of the above, except that I use the AmEx Platinum Card, which I find fits my personal travel pattern (Delta frequent flier, but not frequent enough to have free lounge access) pretty well.