I've created a lesson of 'facts vs. fibs' about Psychology. They have to work in teams to figure out which they think if a fact and which is a fib.... for example "Do we only use 10% of our brain"..... "You are either left or right brained" etc... Then we discuss the real answers- when i've done this before a lot of them are quite surprised/disappointed that they don't have 90% of untapped brain power to use. haha and it helps get them engaged.
Then I lead this into a general "what is psychology and what do psychologists do". I do the classic stanford marshmallow experiment (show them the video in the lesson)..... go through some ideas of what psychologists do, how they conduct research to help them find out things (only very brief so as not to overwhelm them) and then get them to design a poster advertising Psychology to others who know nothing about the subject.
This is for a double lesson and as mentioned previously will be quite lighthearted and not in too much depth as it is an introduction but will hopefully get them excited about experiments etc.