Most things from the smallest items all the way to clothing and beach gear can be purchased at one of the many ABC stores. So why not leave some of the bulky stuff behind and just pick up some in Oahu.
Our last trip we decided to pack light since we tend to do "too much shopping" on our vacations and wind up purchasing more luggage to haul it home. We picked up our sun block, beach towels, beach bag and several clothing items at the ABC stores. The prices are cheap so why not.
Miscellaneous: It's always a good idea to do some research on an area before you travel and build a loose itinerary. I have found that the DK books on Oahu and Waikiki/Honolulu are very good as well as the book Oahu Revealed. Most books can be found in stores such as Barnes and Noble, Borders, etc. On the islands the ABC shops carry a variety of Hawaii guide books to peruse if you didn't plan ahead or forgot your guide at home.
Luggage and bags:
We had the least luggage in our group and still over packed...and it was our wedding/honeymoon! Had we not had to bring dress,suit,gifts, we would have only needed one medium or 2 small roller suitcases.
Clothing/Shoes/Weather Gear: Its really casual and laid back. We brought what we thought would be good shoes, and we both ending up ditching our fancy sandles for cheap foamy ones at the ABC Store. They were more comfortable both on the beach and walking around the city. I practically lived in a light linen skirt that I could wear to the beach or out to dinner. A sweater, or wind breaker..if you are going hiking then warmer clothes might be needed.
Toiletries and Medical Supplies: Of course sunscreen..and probably aloe gel for when you don't re-apply your sunscreen often enough.
Photo Equipment: We stocked up on memory cards and batteries before leaving home..only to find them actually CHEAPER at convience stores in Waikiki. (not something I would count on, but if you leave something at home, you won't be gouged at the check out.
Camping/Beach/Outdoor Gear: Everthing is pretty reasonably priced at stores. If you are going to do much snorkling, you probably want to bring your own gear, since it's packable. We just bought stuff there, and then passed in on to other kids/hotel guests when we left. We got cheap floaties to lay on for $3+$1 to blow up. My husband gave in and bought a boggie boards for like $20, and got totally beat up in the waves farther out..but he had to try it. There are tons of blow up beach things for the kids, and they are cheap enough that when they pop, you can afford another. Even beach towels are cheap, next time I will leave mine at home and buy one there..both a souvineer and saves space on the flight in.
Miscellaneous: In general even though most things are more expensive in Hawaii, (hotels, food, activities) you will find (beach gear, sunscreen, batteries) are not that bad. If you have never been to waikiki before you will be laugh at how many ABC stores there are everywhere!!! but their monopoly makes everything cheap for tourists!
I had picked up a copy of Frommer's "Hawaii on $35. a Day" (remember, this was in 1987), and used it to do a walking tour on our own around Honolulu.
It had a lot of useful information in it.