Halloween math activities for elementary grades usually involve candy. Class or individual student candy graphs are very popular, for instance. Sorting the different types of candy is another activity that usually makes the top 10 list as well. Adding/subtracting candies are a familiar sight in classrooms around Halloween.
How about using other Halloween subject matter to do math? Graph the Halloween costumes worn to the kindergarten Halloween party. Or,find out who's going to dress as what in your classroom, make a list, group similar costumes, then write that group as a representative fraction of the whole group. Put the sweet tooth aside for a while and try this instead!