Not everyone is used to creative techniques. Before starting a co-creative session, a brainstorming or any activity that includes "non-designer" people you should try to open everyone's mind.
To do so you should first state some basic rules: "there are no bad ideas", "we don't criticize each others", "you can all be creative". All the participants should feel comfortable and free to answer or not if there are questions that bothers them at any point.
Then you can use some games to stimulate creativity. Remember the games you were playing in your childhood: pictionary, miming games, playing with words... For example, you can ask everyone to draw as many things as they can in 5 minutes starting from a square or a circle. Or put everyone in single file, give a word to the first one who has to draw it on the back of the second and the last finishes by drawing what he felt on a paper.
Don't forget the goal is to put the whole team in a good mood, open the minds and show that the tasks they are going to get are not complicated. These games will also play the role of the ice-breaker.