The Wand of Create Wand that I've seen a few times on this board would work... Just guess what types of wand it can create!

Other ideas: This one's nicked, but Rings of Friendship. If you wear one of them, you know there's another one in the world. If you wear both, you know where both of them are.

Everfull Glass: a glass that's full of water. It's impossible to get the water out of the glass, hence the name.