We always plan our trips for October when our son has his Fall break. The weather is usually very nice (although it RAINED a couple of days last year), almost all of the rides are open, the water parks are open and MNSSHP is going on. It can get a little busier during the Columbus day weekend, but it is by no means like the summer crowds.
We will not go this year since we went out to Disneyland in June, but we'll be back in October 2006.