Lucifer: You would try to scare him to see his reaction. He get irritated and scare you to teach you a lesson.
Mammon: He would act all brave and take you to a haunted house. But he would be the one getting scared and you would have to comfort him after.
Levi: You guys would hang out in his room and play all of the halloween events on his games.
Satan: You guys would have a scary story contest. The first one to scare the other wins. He ended up winning.
Asmo: You two would model different costumes for each other.
Beel: You guys would go around town and try all of the limited time halloween food items at different restaurants.
Belphie: Sleep.
Diavolo: Its his birthday! He wouldn't let you leave his side. You guys would spend all day together and have a fun night too.
Barbatos: You guys would make halloween treats and hand them out to trick or treaters.
Simeon: Similar to Barbatos, you guys would hand out candy to trick or treaters. Then you would tell scary stories at night.
Luke: He would be too scared to do anything. But he would put on a brave face and go out trick or treating with you. He would end up having fun but he would eat to much candy and get a sugar rush.
Solomon: He would convince you to try some black magic with him. Let's just say you had to go to Lucifer and Diavolo to help break some curses.