If you are planning on staying until the park closes AND the weather is clear, you can gamble on skipping the first parade and waiting until the last castle show. This will let you avoid most of the crowds. While the first parade is going on is a good time to do Splash Mountain and Big Thunder, as the parade route will temporarily cut off access to those rides for a lot of people.
You can free up a lot of time and avoid even more crowds if you are willing to skip the projection show and watch Hallowishes from Fantasyland. Just remember that there is only one Fireworks show and projection show per night.
Keep an eye on restaurant hours. A lot of food places will close long before the party ends, and you probably don't want to be stuck with cosmic ray's.
If you don't have kids with you and are staying on property, it might be fun to go for drinks before the party starts at one of the monorail resorts. Trader Sam's won't be open yet, but Tambu Longue will be. You probably won't be the only people pregaming in costume.
Unlike a normal operating day, the Emporium will not stay open after the official party ending time, so if you want party merchandise don't wait until the last minute.