I was a teacher for ten years before taking a position at a small tech consultancy. I also encourage you to jump into the substitute pool; however, please understand that as a regular classroom teacher you will have many more tools to manage a high functioning classroom (not least, a regular classroom teacher has established relationships with their students). Substitute teaching is teaching, but there's a reason expectations for what's accomplished during that day are typically quite low. This all will change classroom to classroom, school to school - of course.

I think there are many teachers who would love to have an experienced individual come in and guest-speak about one topic or another, if that's something of interest to people.