Depending on the day of the week, you might have to FastPass Harry Potter. If it's one you really want to see, hit that park early. I've seen it where they hit capacity and don't let anyone into that section of the park. #sadness.
But October should be OUTSTANDING weather for you to be in FL. And yah, they'll be setting up for Halloween Horror Nights. If you're into being scared (and giant, drunk crowds), you should check that out, too
~Emily