QR codes would be a really fun way to indroduce a new topic.  For example, I could use QR codes to introduce questions and answers and students would need to match them.  Or they could be topics for discussion - students choose a QR code and follow the link to their topic.  Really a way to randomly assign topics and a fun way for students to discover them.  Could also be a fun "treasure hunt" way to find answers to an in-class assignment.

Students could use it to introduce questions at the end of a presentation, or to introduce the actual topics of the presentation.  Classmates choose QR codes and follow the links, and that is what is presented.  Might put things in a random order, but would sure make those boring presentations more exciting.