Jack is not dressed up – the only thing that appealed to him out there as being sufficiently sexy was a vampire outfit, and he decided that would be in bad taste given Martha's recent transformation into a vampire, not to mention his current relationship with Spike – so he's just out and about in his normal outfit. Which, after all, is a costume in itself, so yay!
He wouldn't normally get into this sort of thing much, but he signed up for helping set up yesterday so he's kind of proud of the decorations he put up and can be seen going round straightening the odd bat and rehanging the odd cobweb that's fallen down etc. He can also be found hanging around some of the activities (apple bobbing, pin the tail on the Jersey Devil, pumpkin carving, NOT face painting!) to provide supervision where needed.
In the evening, you can of course find him at the dance. He'll obviously be spending some of that time with Ianto, but there's plenty of room for dancing – or just socialising – with other people too.