To review: enchanted items are items that can cast set spells a set number of times per day. There is little information in the document as to how these enchanted items are activated.

I propose: the same way that "imbued" spells can be set to trigger automatically from defined stimulus, so can enchanted spells. For example, a ring of Shield might be set to automatically trigger when an enemy strikes the target with a roll that Shield would block.

Triggering an enchanted item intentionally would be a bonus action, and additionally items enchanted with spells cast as reactions could also be activated with a reaction.