I found this comment tonight while catching up on some reading.  It’s a well-articulated statement of how I think lesson planning, and teaching,  should happen:

But the best classes, for me, always seem to be the ones that go
sailing off in some direction I hadn’t anticipated. I used to worry
about having to pull the kids "back on track." In recent years I’m more
interested in trying to explore with them where the new track is
leading. Truth to tell, a great deal of my lesson "planning" is
actually done after the fact, trying as you say, to figure out, given
today’s surprises, what would be a good thing to do next.

And
all of that connects to the objection that I think we both share to
curriculum design driven by standardized testing. There’s no room there
for side tracks, we’ve got to get to page 48 by Thursday.

The
artfulness of teaching is about knowing when and how to respond on the
fly to things you hadn’t anticipated. And if you don’t provide room for
those things to happen, if you don’t give the students room to make them happen, education devolves into something mechanical and soul-deadening.

    Yep.  Couldn’t agree more.