Abby's (Codng right this time >.> ) Darcy: Came in a costume to match Cisco's, that of Judy Hopps from Zootopia. She has on a grey mask wig, but her skin is so pale she didn't actually need to wear any white makeup to pale herself out because the wig did it for her. The coolest thing about the costume, and it's all thanks to Cisco's engineering, is that the ears are made so that when she moves, they react to the stimuli of her head so if she leans down they move down and so on.
River: still only kind of understands what this is, and as such is wearing one of Simon's labcoats. This was a last minute change, as originally she was wearing one of her dresses with the robe the portal gave her back over it and saying she was going "as Inara," until it was pointed out that only a few people here knew who Inara was. So instead she's going to Halloween as a doctor. She even has a stethoscope around her neck. She's more interested in the dancing and the candy and is... kind of on edge.
Sarah (and the boys): Sarah and the boys are dressed up to match this year. The boys were having nothing to do with proper suits, so instead they sort of compromised. AJ is dressed up as Frankenstein, Cas is dressed a vampire, and Sarah has worn a hooded robed dress, with a simple silver domino mask. The boys are technically wearing suit-like things and she is wearing a dress.