I lived on Oahu for 10 years so I think I can shed some light here.
I guess it all depends on what kind of vacation you're looking for. If you are looking for night life, shopping, movies or big city activities, I would say go to Oahu.
The thing is, is that you can probably do all that stuff where you live. If you want to get away from the rat race and do some great outdoor stuff, and get the island feel, I would go to Kauai or the Big Island. You can do all the things Jake7 mentioned, and more, without all those people on Oahu. And if you really want to get away from it all you can check out Molokai or Lanai, but for most people that's a little too secluded.
If you're not really the outdoor type and still want to go to Hawaii I would say stick with Oahu. I'm not saying you can't enjoy the outdoors on Oahu (because you can). I'm just saying you can enjoy the outdoors with a lot less people on the outer islands.If you do end up going just make sure to bring plenty of sunscreen