We are two adults and have been to
MNSSHP for the last few years. It is a lot of fun and there are lots of adults on their own trick or treating through the park. A lot of people dress up but not everyone and it certainly isn't required. When you enter the park you're given a treat bag and there are locations throughout the park where you can go to get treats -- everyone is allowed to collect the candy - this is not just for the kids!
Most of the rides are open and since it's a hard ticketed event the park is potentially a lot less crowded than it would be normally. Most of the rides are walkons or just have a short wait. There is a special parade and special fireworks. The best part is that the characters are dressed up in costumes as well! It's neat to see that -- one year Chip and Dale were dressed as ghosts!
The park is also dressed up for Halloween so it's a lot of fun.
Fiona