The castle stage show, the fireworks, and the parade are "don't miss." Since the fireworks happens once per night, plan around that. The parade goes off twice and the stage show three times. Shows have fewer in attendance as the night goes on. The stage show is, IMO, worth seeing twice (it's not like you can see it on your next regular MK trip, so, soak it in).
There are exclusive items for the night: Sorcerer's card (get it early in the evening before the firehouse closes) and merch and food that they sell only at the party. Some of that is EBay-able if you have the inclination.
There are rare M&Gs, but their lines are long, so, line up early or skip altogether.
Rides *may* have short lines. Depends on whether they sell out or if the crowd is all thinking the same thing: let's go on the rides since they'll be short lines! Oops.
Your Halloween ticket allows you to enter the park at 4 PM though the party starts at 7 PM.