Army Dog tags on Ball Chain
Traditionally used for getting injured or dead soldiers home, you could apply the same to a drunken night.
When you wake up with no idea where you are or even who you are, just point to your tags.
The police will then get you back to the right place.
Or they'll just drop you off at the nearest army base and you'll be done for desertion.
Either way, add a set of these to any soldier costume and you're good to go to war.