Hello Jacqueline - It could be useful if you could give us an indication of the sorts of things you're looking at replacing? I have to say, as an old fuddy-duddy, we don't do too much differently at Halloween.
It can also be useful to google "low carb whatever recipes" - I have found some great alternatives that way. As some returns will be from US sites, you could also search on trick or treat recipes.
I could see a sweet version working too - LC pumpkin pie, with whipped cream (or vegan substitute ), or a LC choc gateau, with whipped cream (?) and glowing red cranberry eyes...