For a first timer I would say the must dos are the parade and fireworks. I also recommend getting the Photopass Magic Shots next to the Haunted Mansion, they are only available at the parties. Doesn't mean you have to buy them, but they are fun to check out once you get back home.
Since you're not going until Oct, you'll have a chance to see what characters will be out this year after the first party (several changes are happening, Disney has already announced that Jack/Sally are replacing the Phineas/Ferb M&G and Sulley/Mike are replacing the Stitch dance party). Take a look at that list, there are several characters that only appear during parties (Tarzan, Jane, Sparrow, Dwarfs) and make sure you hit the ones that you want to!
Avoid the Castle Show to avoid the villains, or watch it from afar (it's a great show). After the first party, reports will indicate where other villains will be located.