we went in sept 06 - oct 06 and the where still quite a few people around, but not as many as the peak seasons. we had a gr8 time and didnt have to wait that long at all for any of the rides, i think 45 mins was the longest we waited but that was the new roller coaster. thought that was good, cant wait to go back.
i would say any time in october would be good, check out the weather over the past few years that might help you decide.
happy planning