I remember one of the exercises we used to do with pharmacy students in Brazil... that was a good example of how to get students more reflexive about the social constructs of reality that influence a lot the way we perceive (or do not perceive) things in the world.
So, "the observer" was an activity where we asked students to go to a common place - usually a place that they were very familiar with - and observe it for one hour, registering everything that they could see, listen to, hear or smell in the scenario. The discussions in classroom always were fantastic! They usually would come up with things that they never observed before, in the everyday life.
This very simple exercise helped us a lot to discuss how the way we see things are, for so many times, "oppressive" to the different realities presented to us. I recommend this activity as a good one for a critical approach in classroom!