Cost saving tips
Hawaii is a very expensive place to live or visit, so I wanted to give some random cost saving tips that may help make your trip more affordable. As a Hawaii friend said, this is my "How to be Chang (frugal) in Hawaii" page. Yes, I'm chinese and proud of it! :)

Costco
A great place to hit at the beginning of your trip to get everything from snacks, Hawaiian Sun drinks (canned tropical fruit flavors) and that tub of kalua pig that you and your arteries have always wanted. The selection at Hawaii Costcos is very different from the mainland and it's a great place to get gifts to bring back home like a 6 pack of chocolate macadamia nuts, tub of individually wrapped macadamia nuts, family size of li-hing mango or 6-pack of hurricane popcorn (popcorn with seaweed, sesame seeds and Japanese rice crackers). Some friends even buy their Hawaiian jewelry from Costco. :) You can also get some brand-name Hawaiian shirts and snorkeling gear.
Note: The Costco closest to town also has a gas station but lines can get quite long.

Longs Drugs
For some reason Longs is more popular in Hawaii than in Northern California where it started. We go here to buy everything from greeting cards, to random food stuff (Spam, saimin, lau lau, Ted's pies) to slippers (aka flip-flops). If you need something and you see it here, it's probably a decent price. There is one in Ala Moana Shopping Center, next to Sears.

Wal-Mart
Yes, all things being equal I would love to support a Mom-and-Pop store, but you can't beat the prices at Wal-Mart.

Swap Meet
Held every Wednesday, Saturday and Sunday at Aloha Stadium (site of the Pro Bowl). Haggle for and buy anything from tshirts, magnets, beach towels to crack seed (dried Chinese fruit and nut snacks). Wear comfy shoes and a hat and bring water, it's huge and gets hot. You'll find most things that the International Market Places sells, only it's cheaper here. They charge a small entrance fee and parking fee.
Swap Meet link

Water
Hawaii can get really hot and humid during the summer and I recommend bringing water with you when you sightsee. However, the tap water in Hawaii tastes great and is naturally filtered by lava rock. So if you are ok with refilling bottles and the water in your hotel room tastes good, you can save some money and the environment by drinking the tap water.

Military Discount
If you have a military ID, you can use it at a lot of places to get a military discount. They are usually the same discount that locals get. The local rate is called the kama'aina rate, but usually you have to show a Hawaii driver's license. Usually these discounts are for hotel rooms, rental cars and sightseeing admission fees.