"Hanging out" might not sound like much of an activity to you, but social interaction is paramount to tween development. Tweens are developing their personalities at this stage of life and want to be accepted by their peers. So, friendships should be cultivated and encouraged. Of course, we're not talking about just hanging out on a street corner. Set up tween "play dates" -- let your tween have a friend over after school, and get to know the friend's parents so they can reciprocate.
If your tween seems to have trouble making friends, help her or him find a club or after school activity to join. This will give your tween something to share with others and also provide some structured social time.