My husband and I don't dress up in traditional costumes. Last year we got t-shirts. His said "I'm the trick", and mine said "I'm the treat".
They were so easy last year that we are doing the same this year. His is going to say "667 I'm the neighbor of the beast" and mine will say "333 I'm only half evil"
They are just t-shirts that can be paired with shorts or jeans. You can get them in long sleeve t-shirts too, but since we are going in September, I think the short sleeved ones will be fine!