Final answer:
To highlight key ideas in a routine message, use numbering or bullets for effective organization and to draw attention to important points.
Step-by-step explanation:
When drafting a routine message to highlight key ideas, you might use numbering or bullets. These organizational tools help to break information into readable chunks and draw attention to important points. In contrast, using external links may not be as effective in a routine message where the goal is to convey key ideas quickly and within the body of the message. Therefore, both numbering and bullets, but not external links, would be appropriate strategies for emphasizing key points in a routine business communication.