Potions, Doppelganger, etc playable as instants?

[MazeMonster]

Do cards like potions, Doppelganger, etc. need to be equipped in front of you to be playable in combat, or can they be played as instants from your hand. There is a player in our group insisting on the latter.

[MunchkinMan]

All one-shot Treasures can be played from the hand, and also from the table (as long as it's an Item, and thus can be carried).