This will be my 5th trip to Hawaii coming up this August. I have been in late April/early May, July, over Labor Day and mid-late October. The weather was great on each of those trips so I can vouch for October being good. IMHO it is much too far to travel and WAAAY to much money for airfare to not go for at least 10 nights. I also think it's a mistake not to visit at least 1 more island, especially if it's your first time. Each island is different and has its own personality. We always visit 2 or 3 islands. I have never spent more than 3 nights on Oahu as it was always our least fav. before hopping to other islands. So this trip will be entirely different for us. We figured staying at Aulani would be a completely different experience from staying in Waikiki so we're spending a full week there. We're going to plan 2 full days of relaxation at the resort and are going to try surf lessons and visit the North Shore. Things we've never done before. After a week at Aulani (which I think will be plenty) we're going to Maui for a few nights.
Airfare. Wow. It's a lot these days. I booked in January for August (ended up paying $1,074 pp). I had been watching it for awhile so I would know a good deal when I saw it but I didn't want to purchase it until I'd secured our reservations at Aulani. Meanwhile it kept going up and has continued to go up since I booked it. We're starting our trip with 4 nights in CA at DL and the leg that's gone up the most is the one from LAX to HNL. There are still some deals but our first choice of flights (early) has gone up (from $272 to $480 one-way). November may not be as high of a demand as the summer tho.

They say it is typically a little cheaper to fly mid-week (Wednesdays). Set up some fare alerts on kayak.com or airfarewatchdog.com or farecompare.com and get familiar with the fares for your anticipated dates.