We've gone several times in October and loved it. The weather is warm, in the 80s usually and still warm enough to swim. The crowds are low, but tend to get busier around the Columbus Day weekend. The parks close early so you won't have alot of late night touring. If they have e-ride nights, you may want to look into those (they're only offered to people staying onsite).
Mickey's Not So Scary Halloween is alot of fun, I recommend it, we've done it twice. This is open to anyone, but you need to buy a ticket for it. The ticket is in addition to your park admission ticket. There's alot of info on the boards about it.
Happy planning!
Nancy