You would do well to get an opinion from the United flyers over at that forum. 4 award tickets seems like a nice score. You didn't mention your departure airport, which could make a huge difference in a response.
Wow--if your husband *really* wants to go to Oahu, (and it's always nice to go home), work from there.
Can you stay on the Big Island with SIL?
If you can trade RCI on Kauai, what about Big Island? A place like Paniolo Greens has King/Twins/Sleeper sofa. And Volcano Park is a nice bonus.
Other thought would be some creative financing. A 0% credit card for hotel, which would require minimal payments until, say, you get a tax refund and pay it off. Maybe some other *found money* outside of the budget.