Change the main content per controller change

Make a MY_Controller (in application/core).
Create a method in it that renders the pages, the way you want.

PHP Code:

class MY_Controller extends CI_Controller {

    public $data = array();

    function __construct()
    {
        parent::__construct();
    }

    protected function render_page($view)
    {
        $this->load->view('inc/header_sidebar');
        $this->load->view($view,$this->data);
        $this->load->view('inc/footer_sidebar');
    }    

} 


Let all your controllers extend this MY_Controller.
If you want to load a certain page with the default header and footer, just use $this->render_page('viewfile-name');
All the data that you want to pass to the view, can be added to the $this->data array, like:

PHP Code:

$this->data['records'] = $this->sample_model->get_all();