I find that graphing vertical and horizontal lines makes a nice one-day lesson in the graphing unit. Spending one solid period on the idea of these two special cases allows for a lot of discussion of slope and the basics of getting equations on a graph.
The best part of this lesson is that it can be done at any point within the graphing unit - I have done it both at the very beginning or during a shortened Fall Break week when I had an odd time to fill.
This lesson begins with notes about vertical and horizontal lines, allows for some guided practice and then stretches student understanding with a brief exercise. After that, students will do Independent practice before a fun activity called "Find someone Who Can." Students move around to find others in the room that can complete the three tasks they all have. I find it is a good way to get out some energy and have students discuss misunderstandings. Finally, an exit ticket is included to see if students mastered the concept.