Kauai. Plenty of touristy but legit things to see & do any time of year. I prefer late winter/ spring but that's just me & if you're keen on more small town flavor then it's the one. Had a place in Kapaa for a long time so I can't speak from personal experience for staying @ any resorts/hotels but there are some lovely choices.

If you're curious about Oahu, save it for your way out, maybe. Not a bad place just not where Id chose to spend my time & money for a vacation though the memorial is worth the hassle of a day trip.