Important things to remember:
- Monster Enhancers (cards that say things like "-5 to Monster," or "+5 to Monster") are not Items, and can only be played from your hand on a single Monster.
- Items will always have a gold piece value (which includes the words "No Value"), and can always be put into play with the exception that you can only ever have one Big Item in play.
- One-shot Items (which will say "usable once only") have special rules which say they can be used from your hand or from in play. This means you can put them into play, and do not have to use them immediately when you do. Most one-shot Items are to be played to give a bonus to either side, though there are other effects here and there.
As an aside, there are a few cards, here and there, that do say "usable once only," but do not have a gold piece value. They are not Items as the "usable once only" phrase is not the determining factor on what is an Item.