Yeah, Oahu sucks. If you want to see Oahu, just go to somewhere on the East coast, it will be alot cheaper. The only saving grace in Oahu is for history buffs...if you want to see the USS Arizona.
I haven't been to Kauai, but Maui is badass. I went there a couple Christmas ago and going back for my honeymoon. If you plan on making it over to Maui you HAVE to do the Hike Maui rainforest hike, preferably the half-day. Going to see Molokini crater (snorkel) is also pretty cool, since its only one of 3 in the entire world like it. Do NOT do the road to Hana if you go to Maui. Its a massive waste of a day.
I've heard Kauai and Lanai are awesome, just like Maui. Its actually not that hard to get back from the small islands, if you're willing to pay for a short 15 minute plane flight over and back. You can buy the tickets at the counter, they're never, ever even close to full.
If anybody needs any info on this stuff you can PM me as well.