You don't even really have to qualify what "grade" you are (for want of a better word). I'm sure it'd be okay to say something along the lines of "I like to relax by going on long walks, and am an active member of the Duke of Edinburgh scheme. This has helped me to develop my confidence and my ability to work well with others". You'll want to to reflect in a more sophisticated way, but I'm sure it'll be fine to phrase it like that, rather than explicitly saying you have a Silver Award.