public function addons(){
    return $this->belongsToMany( 'AppModelsAddon', 'na_activity_addon', 'activity_id', 'addon_id');
}

public function activity()
{
    $this->belongsToMany('AppModelActivity', 'na_activity_addon', 'addon_id', 'activity_id');
}

$activity = Activity::findOrFail($id);
$addons = $activity->addons->all();