I have in my Model class:
Mage::getDesign()->setArea(Mage_Core_Model_App_Area::AREA_FRONTEND);
$layout = Mage::app()->getLayout();
$block = $layout->createBlock('core/template','blockname');
$block->setTemplate('cronjob/items.phtml');
$blockProducts = $block->toHtml();
If I use these, Magento warns me with the message:
CRIT (2): Not valid template file: frontend\base\default\template\cronjob/items.phtml
File items.phtml is at these path: C:\wamp\www\magento\app\design\frontend\default\customtheme\template\cronjob\items.phtml