You won't find liquor stores within walking distance unless you're in a resort with an ABC store with double the prices.
Kauai is like a small town and at 10 PM its lights out for pretty much the entire island. If you stay up past that you'll likely get yelled at by somebody. Also most places there other than resorts don't have AC its too expensive. There are some great beaches up around Princeville.
Maui has awesome beaches and lots to do. Since you a diver look into snorkeling the Molokini crater the water is so clear you can see 200 feet away. I'd stay away from Makena they have a shark there that eats people.
Oahu is all over the map price wise, great beaches that are easy to visit Monday - Friday. On weekends when the locals have time off you might find it very hard to park at some of the best beaches. Whatever you do, never book on the west side of Oahu unless you want to risk getting into a fight.
There was rumors on the island a nuclear attack happened recently and the missile was shot down and they covered it up or word of that got out it would kill the tourism industry.
Don't bring snorkel equipment since you're probably limited to 50 pounds of luggage just rent it from one of the snorkel shops. You'll want to stay atleast 10 days because 2 days alone are blown travel to and from. You'll save big bucks if you book your rental car, place to stay many, many months in advance. Infact some of the best rentals on the islands are well known by repeat costumers and the best rooms for the best prices are typically booked 10 months in advance. Sure you can find something say next week but you'll pay a fortune.
Also for a free ticket go to
https://www.hawaiianairlines.com/ and both you and your better half sign up for there credit card. After you're approved you'll get 15,000 miles for signing up. Have your partner do the same and forward the 15,000 miles to your account. I believe with 30,000 miles that will qualify you for 1 free ticket round trip that will save you about $700.00
There are lots of free or almost free things to do. Pearl Harbor is cheap and a must see. The beaches, hiking, sight seeing, people watching, whales at this time of the year are free.
DON"T pay a boat to go out and whale watch. By law they have to stay half a mile or so away. Just hike up some trails and you'll see whales all over for free. There are said to be 10,000 whales in the area at this time of year.
Believe it or not there are not many good places to eat on most of the islands. Forget a good burrito it can't be found there, nor can jumbo shrimp unless you go to bubba gumps. Good steaks are hard to find, places like Kauai only have a handful of places like that to eat and it won't be cheap.