Ideas for a short session on Assertiveness

Hi - I am running a 1 day session on preparing for the job market. I would like to include a brief overview of assertiveness skills. Does anyone have any effective games/activities to introudce this session with?
I will probably spend only 20mins maximum on assertiveness during the course but want to make it interactive rather than just talking to them about. The group is of unknown ability/background and I won't know how many people there will be until they turn up.
Thank you...

 

re: my post - forgot to mention that they will be adults attending the session (but unknown ages)