hidden-toggle for cmb. Works like the WordPress metabox toggle (but for only a selection of fields)

cmb-toggle-fields.css

.advanced-toggle .toggle-label {
  cursor: pointer;
  display: block;
  line-height: 3em;
  border-bottom: 1px solid #e9e9e9;
  border-left: 1px solid #e9e9e9;
  border-right: 1px solid #e9e9e9;
  padding-left: .8em;
}
.advanced-toggle .inside .cmb-row {
  padding: 1.4em 0;
}
.advanced-toggle:before {
  display: block;
  float: right;
  clear: both;
}

#normal-sortables .advanced-toggle .inside {
  padding-top: 1em;
}
#normal-sortables .advanced-toggle .inside .cmb-row {
  padding: 1.8em 0;
}

.js .meta-box-sortables .postbox .advanced-toggle.closed .handlediv:before {
  content: \'\f140\';
}

cmb-toggle-fields.js

jQuery(document).ready(function($){
	$( ".advanced-toggle" ).children("small.toggle-label, .handlediv").click(function(){
		var $this = $(this);
		var $parents = $this.parents( ".advanced-toggle" );
		if ( $parents.hasClass( "closed" ) ) {
			$parents.removeClass( "closed" );
			$this.siblings( ".inside" ).show();
		} else {
			$parents.addClass( "closed" );
			$this.siblings( ".inside" ).hide();
		}
	});
});

cmb-toggle-fields.php

<?php
if ( file_exists(  __DIR__ .'/cmb2/init.php' ) ) {
	require_once  __DIR__ .'/cmb2/init.php';
} elseif ( file_exists(  __DIR__ .'/CMB2/init.php' ) ) {
	require_once  __DIR__ .'/CMB2/init.php';
}

add_filter( 'cmb2_meta_boxes', 'cmb2_sample_metaboxes' );
/**
 * Define the metabox and field configurations.
 *
 * @param  array $meta_boxes
 * @return array
 */
function cmb2_sample_metaboxes( array $meta_boxes ) {

	// Start with an underscore to hide fields from custom fields list
	$prefix = '_cmb2_';

	$hidden_open = '
	<div class="advanced-toggle advanced-toggle-wrap closed">

	<div class="handlediv" title="' . __( 'Click to toggle', 'cmb2' ) . '"><br></div>
	<small class="toggle-label"><span>' . __( 'Toggle Advanced Options', 'cmb2' ) . '</span></small>
	<div class="inside">
	';
	$hidden_close = '</div></div>';

	/**
	 * Sample metabox to demonstrate each field type included
	 */
	$meta_boxes['test_metabox'] = array(
		'id'            => 'test_metabox',
		'title'         => __( 'Test Metabox', 'cmb2' ),
		'object_types'  => array( 'page', ), // Post type
		'fields'        => array(
			array(
				'name'       => __( 'Test Text', 'cmb2' ),
				'desc'       => __( 'field description (optional)', 'cmb2' ),
				'id'         => $prefix . 'test_text',
				'type'       => 'text',
			),
			array(
				'before_row' => $hidden_open,
				'name'       => __( 'Some advanced feature to be hidden by default', 'cmb2' ),
				'id'         => $prefix . 'advanced_1',
				'type'       => 'textarea_small',
			),
			array(
				'name'      => __( 'Another advanced feature to be hidden by default', 'cmb2' ),
				'id'        => $prefix . 'advanced_2',
				'type'      => 'text',
				'after_row' => $hidden_close,
			),
			array(
				'name' => __( 'Test Text Small', 'cmb2' ),
				'desc' => __( 'field description (optional)', 'cmb2' ),
				'id'   => $prefix . 'test_textsmall',
				'type' => 'text_small',
				// 'repeatable' => true,
			),
		),
	);

	// Add other metaboxes as needed

	return $meta_boxes;
}

toggler.gif