This is the one night adults along with children are welcome and even encouraged to dress up in Halloween costumes! We always have a lot fun doing this! A lot of people who have never attended ask if adults end up dressing up and a fair amount do (at least half). So if you are hesitant or worried about looking out of place don't worry you won't! You don't have to dress up and some people don't. But I would recommend it because it will only enhance you experience.
When you enter the party everyone is given a bag to use to trick or treat at designated candy spots around Magic Kingdom which again is for both children and adults! This is pretty much the only place adults can trick or treat where it is encouraged! Many adults (myself included) will be stocking up on candy as well! We generally don't end up eating it all, but it is a lot of fun! If you don't eat all of the candy you can always use it to hand out at home on Halloween. The candy you receive is a pretty good mixture of standard candy. Some locations will have special treats like Werthers chocolates in Fantasyland. So have fun!
Throughout the evening there are several dance parties going on around Magic Kingdom which you can stop into at any time. There is a Villain Mix and Mingle show that you can see several times during the night which is very popular and you can see many of your favorite villains on stage in front of Cinderella Castle. You also will find many of your favorite characters around the parks for meet & greets along with would recommend some unique characters which are not in the parks normally. I prioritizing which characters are the most important to meet because the lines can be long for the popular one, so you will not have time to meet them all. The most popular are Jack Skellington and Sally from The Nightmare Before Christmas and the Seven Dwarfs. The lines for both can be hours long! I have never bothered with the seven Dwarfs but this year we did wait for 1.5-2 hours for Jack and Sally at the front of the park. Mickey Mouse and some of the Princesses don't often have long waits.
The Halloween party also has Mickey's Boo To You Parade which is unique to the party. There are two showings for the parade each evening. I would recommend attending the second one because it is much less crowded than the first one. This is one of Disney's best parades and I would highly recommend going to at least one of the showings. Sometimes we end up watching both, the first from behind the crowd and the second from a location on Main Street. This is where I would recommend finding a place if you are going to the second one. Of course this is also where the parade ends so it will take it longer to get to you but I find with the lighting and atmosphere it is my favorite place to watch. One of the biggest things at the parade is the headless horseman who runs through the parade route before the parade. He is very fast so be prepared with your camera (and you still will likely get blurry pictures).
And finally the best part about the Halloween Party is the nightly fireworks show Hallowishes. This alone makes the party a must do because this is one of Disney's best fireworks shows! In my opinion (and most) it is even better than their regular Wishes shown nightly all year long. So this is an absolute must do! The soundtrack is fantastic and the actual show is amazing. I would highly recommend seeing Hallowishes at least once in your life :)
Remember to prioritize your night because it is really difficult to have time to do everything. That's why I always try to get to the park by 5pm to make the most of our time. We eat and get on a couple of rides before everything starts like mentioned earlier. This year we were in line for Jack and Sally before the party start and then went through to park to all of the candy spots! There is a card game in the park called Sorcerers of the Magic Kingdom which will have a card that is unique only to the party and it changes yearly so I always make sure to get one of those early in the night. We also just watch the Villains Mix & Mingle before Hallowishes to make the most of our time!
So if you are thinking about attending I would highly recommend it and have fun!