Your issue is not unique because red eyes back to the mainland are so common. I've often wondered why some entrepreneur hasn't started a day use lounge/pool/restaurant near the airport, with huge lockers for people to stow their luggage, so that they can enjoy that last day. (Think of lockers at ski resorts or DisneyWorld, but make them a lot bigger and $20 or so.)

Here are some options:

1. Rent a room for an extra night
   
   This is by far the easiest option, though the most expensive. If you have points, this a great time to use them. But I can't stress how nice it is to not have to rush and check-out by 10 or 11am, only to be orphaned the rest of the day. You'll have a place to hang out, shower and change, and secure your luggage during that awkward period.
   
   And it frees you up to do whatever you want that last day. (Hint: it's a great day to take a Pearl Harbor tour.)
   
   
2. Use courtesy lounges/suites, if offered by your resort.
   
   Hilton Hawaiian Village* on Oahu, for one, lets registered guests make a 1/2 hour reservation the day of their check-out in a "courtesy suite," which is basically an unoccupied hotel room, so that you can shower and change. If I recall correctly, you have to wait until the morning of check-out to sign up for a time, but if this interests you, call the hotel directly and ask about it.
   
   
3. Other resorts, especially condos/villas, have a courtesy lounge, which is similar to an airport lounge, except it has showers, too. No reservation required. The Westin Ka'anapali timeshare villas in Maui offers these, but I don't personally know of any on Oahu.
   
   Both options 2 and 3 allow you to do what you want during the day, including sweating/swimming/snorkeling, because you'll have an opportunity to shower and change.
   
   
4. Use the resort's outside shower/bathrooms
   
   If your hotel doesn't have courtesy suites, ask if they allow guests to use their facilities after check-out. Most will allow this sans room-charging privileges. If that's the case, and they have an outside shower/bathrooms, you can accomplish the same thing as options 2 and 3, albeit with less privacy.
   
   If you're doing options 2-4, I'd advise having a separate bag containing toiletries, a large garbage bag to store wet clothes/snorkel gear, and a set of dry clothes.
   
   
   Other options:
   
5. If you don't want to return to the hotel, simply throw your luggage in your rental car (if you have one) and head out to tour the island, lunch, etc. Just take care not to get too sweaty or wet. We've done this a few times.
   
   Of course, keep all valuables (laptops, cameras, wallets) in a backpack to take with you. We leave the doors of our rental car unlocked to show potential thieves that we're not worried about them stealing our underwear.
   

*BTW, as a mere silver elite member, HHV upgraded me to an incredible oceanfront corner room in the Rainbow tower, plus they threw in free breakfast. So I highly recommend this resort. I asked for a late checkout, but the best they could offer was 1pm.