Many magic items require an action (or bonus action) to be properly activated (or deactivated).
But there is a common magic item in Xanathar's Guide to Everything (p. 138) that doesn't seem to require an action to (de)activate: the Pole of Angling.
While holding this 10-foot pole, you can speak a command word and transform it into a fishing pole with a hook, a line and a reel. Speaking the command word again changes the fishing pole back into a normal 10-foot pole.
Let's say Bob has a Pole of Angling with the "BEE" command word. He holds it in his hand, and starts saying "BEE BEE BEE BEE BEE BEE BEE BEE BEE". Will the Pole transform again and again and again in the span of a few seconds (a single "turn")? Or does the command word implicitly require an action?