I walked through the ways to draw ray diagrams with lenses with my classes. The last few years, I just told my student, "eh, it's basically the same thing. Let me show you how to draw the three principal rays, okay, do the rest on your own" and it's kinda been a disaster. And when I went through this, I asked both classes if they were bored. They weren't! I guess it's worth the time in class.
Then we had a sunny day and burnt paper with mirrors and lenses. It was great to watch students figure out how much more difficult it is to burn paper with mirrors. I especially liked it when the kids who had the convex mirrors figure out why it was so hard to burn paper; I mean, the convex lenses worked, right?