The schools I'm at right now don't have a lot of class time to spend on Halloween, so I've mostly been coming up with little things to do outside of the classroom. Even if you do have time for activities, these are fun little supplements to add to the Halloween spirit! So to speak :)
Trick or Treat Janken
-Prepare: simple prizes (I use stickers)
-A game to play between classes. I tell the students to say "trick or treat" if they see me in the hallways. First I ask them a simple question - Halloween trivia (when is Halloween? What color is a pumpkin?) or simple conversation (what sweets do you like? Do you like horror movies?) - then once they answer, we play rock-paper-scissors. If they win, they get a prize.
-I think this one works better for JHS and haven't tried it in ES for fear of the chaos that might ensue. Really simple but the students get excited about it!
Choose the ALT's Halloween Costume
-If you're into dressing up, it could be fun to have the students vote on your Halloween costume! Before and after class and during recesses, I go around with a poll and ask the students to help me choose. It makes the reveal more personal and creates nice chances to chat.
-I usually give the students a pre-set list of costumes to vote on to keep things easier for myself, but you could up the ante even more by collecting ideas at the start of October and running the poll afterwards.
Monster Party
-Prep: a larger piece of paper
-If your students like drawing you could take a drawing around for everyone to add on to during breaks! I like to draw a simple party room and make it look like music is playing, then ask the kids to draw monsters. Together you can make a monster party!
Scavenger Hunt
-Prep: paper/printed Halloween characters, simple prizes
-I haven't tried this one out myself, but you could do it as a longer Halloween challenge. Ask for permission first, and hide Halloween characters around the school. Advertise it on your English board or at the start of class, then have students look for them and report back to you for a prize.
-You could also keep some elements of the targets a mystery and have the students report them to you - "three pumpkins", "orange cat", "Pikachu ghost".