Just looking through costumes in a catalog or wearing one will put you in the spirit.
Watching movies is fun. Movies have always had a way of getting me into the holiday spirits. If you like Horror, you could pick from plenty of horror movies.
Halloween Theme TV Shows
Shows like Family Guy, The Simpsons, SpongeBob Squarepants and many others produce TV shows that feature Halloween. Scooby Doo is a TV show where it's always Halloween.
Trick or Treating is where kids get candy and adults give it out. Maybe you can buy tons of candy to give out to the kids if you're an adult.
Or creepy music will do. Music is a powerful tool to get someone into the holiday spirit.
Decorate your house or your room with Halloween decorations such as spider webs, Halloween posters, pumpkins, skeletons, and other creepy looking things.
These can also be used as decorations and I don't recommend eating them afterwards.
Take a friend or a relative to a pumpkin garden and pick pumpkins with them.
Throw a Halloween party or attend one. Or both.
Go there if you'd like some adventure.