Meatballs in a crockpot: bag of readymade meatballs (Italian style or whatever, I've even used turkey meatballs), one 24 ounce jar of grape jelly and two 12 ounce jars of chili sauce (in the same area in the grocery as ketchup). Dump all of it in the crockpot, cook for 3-4 hours on high. If you throw it together in the break room in the morning, it should be good by lunch time.
Bring a large spoon or tongs for fishing them out of the crock.
But you can always bring grocery store bakery Halloween cookies or some deli stuff if you are pressed for time.
I always make deviled eggs. They're ALWAYS a big hit. Since it's Halloween, I use green food coloring in the yolks and joke that I thought the party was a week earlier.
My sister makes a killer veggie pizza (basically, lay out a can of that crescent roll stuff, make some good ranch ((dye it for fun!!!)), and lay all kinds of vegetables on it. She uses shoestring carrots, broccoli, cauliflower, maybe tomatoes and peppers, and shredded colby jack cheese). It is a MAJOR hit every time.