Hmm I think the box idea is treading the fine line between original and silly.. so I wouldn't go for it personally, it might just look like they fell into a box lol
As for your costume - out of those 3 I would say, go for the witch. It's the most fitting with Halloween and you can go in different directions with it - you could be a traditional "Wicked Witch of the West" type witch with the green facepaint and crooked nose, you could be a "good" witch and dress up in a pastelly, pretty dress and a garland of flowers, or you could dress up as a witch of (..) spiders or bats or something spooky. For Witch of Spiders, you could cover your dress and hat in cobwebs and wear spider jewellery, or for Witch of Bats, you could stick little bats to your hat and wear a mask. Be creative and you can achieve anything
