September is slower than summer. But later September sees more people because of people coming in for the Ironman. Once the Ironman is over travel drops down until Thanksgiving. Then it is slower until just before Christmas then picks up for the holidays and whale season. This per the concierge at the Royal Waikoloan (now the Marriott Waikaloa) She stated that they closed down an entire wing of the hotel after the ironman from October until late December. This was pre-Marriott so I don't know if they do that now, but it is a good indication of the slower time.
Our first trip to Hawaii was in 1995. It was the first two weeks of October. We ran smack into the Ironman on the Big Island (not knowing it was during that time,) and then traveled on to Kauai and Oahu. The convention I went to was in excess of 11, 000 people attending. In traveling through the islands during that period it was funny to hear the locals trying to figureout why there were so many people there during the off season. So I would say October is definately a good time to travel to the islands.