A BOO to you! I posted this before and went and dug it up for you...
MNSSHP: luv's Not To Be Missed List...
1. Headless Horseman - he rides before the parades and may be the highlight of the night. (Doesn't ride if the ground is wet and not safe for the horse.)
2. The Boo to You parade. Try to get a good seat. By "good", I mean on the curb or at the rope. No one place is much better than any other, IMO. But it's easier and more fun if you can see the Headless Horseman and parade stuff easily.
3. Fireworks. The fireworks themselves aren't all that amazing, but they're fun and the soundtrack is awesome.
4. The Haunted Mansion. The ride is the same, but there is a ghost who will chat with you as you enter the line and the staff are all decked out. Pretty cool.
5. The barbershop quartet. They're dressed for Halloween and sing Halloweeny songs. One of my favorite parts.
6. The Villains Mix n Mingle. I don't go meet the Villains. If you want to do that, be up front and ready to book over to them when they come down. They don't stay long! But the show is good.
There are also trick-or-treating places for kids and adults alike (lines get shorter later in the night), sometimes there are candy trails (the one that begins in Fantasyland and ends by Space Mountain is used more often than not), multiple character meet n greets (e.g. Seven Dwarfs! All together!), dance parties for little kids (or anyone, really) and party-specific stuff to buy, if you want a special souvenir of the event, like a pin or whatever.
Tickets - Your ticket gives you access to everything going on in the MK. The party is a park-wide event and your ticket gives you access to all of it. I don't buy mine until the day of the party because I won't go in the rain. But if you put off buying a ticket until the day of the party, you risk having them sell out. It's a hard choice to make.
Wristbands - they'll give you one when you enter, if you enter around party time. Multiple places where you can pick one up if you've been in the park during the day. You'll need to show your wristband periodically to the wristband police, who set up human blockades to prevent people who didn't pay for the party from being there. They even interrupted us while we were eating once and demanded to see our wristbands.
Maps - there are special park maps, just for the party. It lists the rides and places to eat that are open during the party, shows you where the Trick-or-Treat spots are and has the all-important parade, villain show and fireworks times. They're cure, too.

Pick one up!
Weather - it can be very hot (especially September or early in October) and it can get cold (especially later in the month).
Costumes - No way to predict temps, so if you have costumes, try to make them adaptable to any temp. Because that's so easy, lol.

Adults can dress up, too (but it isn't required. Some do, some don't. Whichever you choose, you feel out of place.) Disney frowns upon and may not let you wear masks that will obscure your vision, so leave those out. It is the only time of year that adults can go to the parks in Disney character costumes and many do. You can get spooky with the costumes, but not too gory.
Although the party begins at 7:00, they usually allow people with just a party ticket into the MK around 4:00.
If you have an AP that gives you free parking, it doesn't cover MNSSHP. So, be sure to get through the toll plaza before 7:00.
If you all are into the horror part of Halloween, check out Halloween Horror Nights at Uni. Tons of fun...but not for children!
Have a great time!!!!
Happy Halloween!