Oahu - Honolulu is awesome. The shopping and zoo are fun. Exotic birds in the cages, and just flying around like normal, lol. Everywhere is expensive except the convenience stores. The Dole Plantation is worth checking out, as is the North side of the island for the surfing and shrimp trucks. I'm sure we missed a bunch, like the whale sightseeing (wrong time of year). Beach parks around the north side are ok in daylight.
Kona (the big island) - Because the mountains in the middle stop the rain, the west side is barren desert, and the east side is lush tropical. Weird. Great snorkeling on west side. Nice little beach towns in a few places. The historical parks are interesting. Coffee lovers should check one of the coffee farm tours. A helicopter ride around the north cliffs is fun. Volcano Park has a nice small visitor center by the entrance, sulfur grounds, steam vents, places to walk among volcanic rocks of different types, and a nicer vistor center up high worth going to at night so you can see the red glow from the active volcano (Mt. Doom in real life).
I got hooked on Guava juice and Macadamia nuts while there, Kalua pork and Mahi-mahi are good, and yeah try the Loco Moco if you don't have a cholesterol problem (yet). And luaus really are the best dinner theater ever. (The poi isn't that bad.
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As far as flying, going there was funny, only trip I've been on where the stewardesses did special stuff after takeoff: flowers in their hair, Hawaiian music on the loudspeakers, serving juice. It is a long boring flight, but if you can sleep in coach, no problem. Recommend sleep instead of watching movies because the time difference from the mainland is also a *****.