Fireworks: I really like to watch from Main Street. I recommend either being between the Castle and Partner's Statue or on Main Street (from where the buildings start). Wherever you pick, make sure you have a good view of the left side of the castle. You really need to be able to see the area to the left of the third main turret and above the red line in the photo below. If you cannot see the area, then you'll miss out on the "orb", which is where they project Jack's face, Oogie, and other images.
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If you're between the Castle and Statue, that's one of the "prime" spots and they fill up very early. I've never wanted to put that much time into waiting, so I don't really know how early you'll need to get there.
On Main Street, there are two options: you can follow the parade down from the Hub to whatever spot you want or wait somewhere in the crowd on Main Street and move into the street as soon as the parade passes. There isn't much time (~15 minutes) between the end of the parade on Main Street and the start of fireworks, so this tends to work pretty well. Last week, we followed the parade from the hub and then stopped when we got to the Camera Shop, in the middle of the street. There is really no way to know in advance if someone taller than you will end up in front, whether someone will put their child on their shoulders, or if someone will leave a balloon up. Unfortunately, those are just things that happen and there isn't really much that can be done.
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