Mini-site on Sub-Domain
What I would do is create a new content type "Specific page" (id: specificpage).

In the content type, put all the "fields" / content that your client can edit. You don't want your client to break your HTML structure, but he probably wants to edit the title, the content, the background color, etc. Define all the fields he could want to edit and create them in your content type.

Use this code to theme the "page.tpl.php" for this specific content type:

function phptemplate_preprocess_page(&amp;$vars) {
  if (arg(0) == 'node') {
    $vars['theme_hook_suggestions'][]  = 'page__' . $vars['node']-&gt;type;
  }
}


Then create a page--specificpage.tpl.php based on the original page.tpl.php, put it in your theme folder, structure your HTML content and display the fields defined by your client in the back-office.

[Edited the answer bc I did a mistake in the code and template suggestion]