$variables = array('variable_name' => 'Your value');
$variables = array('variable_name' => 'Different Value');
$variables = array('variable_name' => 'Another one');
$this->return_data = $this->EE->TMPL->parse_variables($this->EE->TMPL->tagdata, $variables);
It is important to not manually specify indexes for the $variables array. The above example would loop through your tag pair three times and replace the output. Of course, normally you wouldn't be manually creating each individual iteration and it would come from query results or some other set of data.
For more information on variable parsing, go here.