I have not found any easy way to accomplish to simply check a Checkbox by default. That can not be that hard, so what am i missing?
Use the FormBuilder::setData() method :
$builder->add('fieldName', 'checkbox', array('property_path' => false));
$builder->get('fieldName')->setData( true );
"property_path" to false cause this is a non-entity field (Otherwise you should set the default value to true using your entity setter).
Checkbox will be checked by default.