I gave an exam yesterday in Principle of Microeconomics. For the first question I asked to students to name an opportunity cost. The most common answer was by taking this exam my opportunity cost is sleeping, watching tv, and playing guitar. I also asked my students for an example of an externality. In class I gave an example of a dog food factory that produces a terrible smell that bothers non-dog owners. This actually happens in real life too, Muslims protest over pet food factory that could 'rain down' pork.
Another problem I gave was on pollution permits at London Heathrow airport. Due to noise and CO2 emissions there is a call to reduce the number of flights (link).
Now unfortunately the opportunity cost of me providing more on the blog today is grading exam