They have been very entertaining so far. Student work, educational posters, behavioral bumper stickers (evidently often in vain), all provide nice grist for the ol' mill. Some grizzled old science teachers have very informative ones, especially on astronomical and biological subjects, way more involved than "back in my day." Of course now I'm not only having fun brushing up, but trying to think about how to explain this stuff to the kids (although my opportunities to really teach are few and far between, I like to at least take the opportunity to think about how I might.) I did get to explain what the problem was to a girl who asked me why Australia "is a continent but is not an island". (We just call them that) Sometimes I try to shout over the rowdy kids and talk about the material to the quiet ones who are doing the work, which appears to be better than nothing in some sense, but even they don't seem to get much involved with it. Unfortunately as a temporary visitor I'm not all that much involved in whether they're listening. It's too bad that few people see the substitute being there as an opportunity for different kinds of learning spaces, rather than just an empty space where you "try to fill time productively" with programmed classwork (usually dittos or copying).
I guess they don't think it likely that every substitute will be able to hold forth on any random topic, but I have never taught a class where I wasn't able to look at what they are reading without fully understanding exactly what was going on. There are many ways I could be of service, but it's just not being asked of me. It's too bad full-time credentialed teaching is the only way to get past that. The result I'm seeing is kids wind up taking the substitute being there as an opportunity to do whatever they want, which does occasionally involve some creative social activity in the form of outburst, but seems like such a waste considering how much they could be learning, especially given the possibilities with modern (and even decades-old) tech. Factory workers have always complained that there's no way to "do the job well."