Here's a way you can embed a node in a collapsible fieldset into another page callback:
<?php
function MYMODULE_MYFUNCTION() {
$page_contents = "";
// ...code...
$fieldset = array(
'#type' => 'fieldset',
'#title' => t('MYTITLE'),
'#collapsible' => TRUE,
'#collapsed' => TRUE,
'#value' => node_view(node_load(MYNODEID), TRUE, FALSE, FALSE),
);
$page_contents .= theme('fieldset', $fieldset);
// ...code...
return $page_contents;
}
?>