Auto-Launch Block _toHtml() in Magento

I need to include a block in another block and launch it directly.
What I do now is this code in the first block

$this->getLayout()->createBlock('module/block');

then on that newly created block I have this

public function _prepareLayout()
{
    $this->setTemplate('module/block.phtml');
    echo $this->_toHtml();
}

This works fine, but having a echo in a the block model I don't really feel comfortable.

Is there any proper way to auto-launch this template at call of this block?

== EDIT ==
After trying the solution of @Keyul Shah it doesn't display anything.
So here is a more precise code of what I did.

I'm trying to call the block from a Admin Widget Form, so I put this in my XML:

<reference name="widget_instance_edit">
    <block type="myModule/adminhtml_my_block" name="block_name" output="toHtml" template="myModule/toShow.phtml" />
</reference>

The module is called as everything in _prepareForm() displays, but the phtml is not displayed.
If I try to do echo $this->_toHtml(); this time it has an error because it launches it before the parent.

=== FULL CODE ===
app/code/local/NameSpace/Module/Block/Adminhtml/Module/Images/Edit.php

class NameSpace_Module_Block_Adminhtml_Module_Images_Edit_Form extends Mage_Adminhtml_Block_Widget_Form
{
    protected function _prepareForm()
    {
        // I want to call a specific block on top of the Edit Form
        $this->getLayout()->createBlock('module/adminhtml_module_images_tagger');

        // addFields stuff
        ...
    }
}

app/code/local/NameSpace/Module/Block/Adminhtml/Module/Images/Tagger.php

class NameSpace_Module_Block_Adminhtml_Module_Images_Tagger extends Mage_Adminhtml_Block_Template
{
    public function _prepareLayout()
    {
        // This works but is ugly and want to change it in a correct thing
        $this->setTemplate('module/tagger.phtml');
        echo $this->_toHtml();
    }
}

The tagger.phtml doesn't matter.

=== Revision ===

After much more experience in Magento, I come back to this question and I realize that I was just on the wrong path.
The best way to do it is to create a block extending Mage_Adminhtml_Block_Widget_Form, then simply call $this->setTemplate('path/to/your.phtml'); and then get inspired by the phtml report/grid/container.phtml