Other Treasure cards (like Go Up a Level cards) are not Items. Most of these cards say when they can be played, and whether they stay in play or are discarded. A couple of specific examples:
Go Up a Level cards may be played on yourself or any other player at any time, even during combat. Discard them once they are played. Exception: You cannot play a Go Up a Level card to give a player the winning level.
Hireling may be played at any time, on any turn. You cannot give a Hireling an Item to carry while you are in combat, however.