Can't you use:
@section('og-image', URL::to('/').'/assets/img/activities/path-to-image.jpg')
?
But you have access to those variables by using:
Request::server('SERVER_NAME');
So it would be:
@section('og-image', 'http://' . Request::server('SERVER_NAME') . '/assets/img/activities/path-to-image.jpg')