Well, although this does not fit in the mathmatic category, I will still throw out some ideas for you.
- All dress up like Minions
- If your into it, you could all dress as superheros.
- All go as a different flavor of LaCroix (Costumes can be bought online)
- Try all going in color matching suits.
- All go as disney characters.
The list goes on :) Hope I helped a tad!
