Playing the Devil's Advocate is a great way to teach wariness to just taking idea's at face value, and will also develop your child's ability to work out why the statements are wrong. Another practice that me and my mother would do is have a conversation, and let it flow to where it may. At some point, we would then stop the conversation and try to follow the flow backwards to the original point of the conversation. While it sounds mundane, I now look back on it as great practice in following my train of though, and seeing why I think what I think.