
Penny & I have worked on several codesign projects and have discovered the planning process is critical. Creating inspiring, playful and stimulating materials to enable participants to connect and communicate their behaviours and needs takes time and ultimately lots of experimentation. People generally don't feel comfortable with a blank sheet of paper yet too much structure e.g. a detailed template can direct and narrow the focus.
Gamestorming is a great resource to use to get thinking about potential activities - lots of great ideas that can be easily modified to suit your project needs.

Here is a little cheat sheet
http://www.selflearners.net/Notes/Gamestorming