Separate menu item for certain pages

I’m working on a site where I need to implement microsites, which have a completely different design than the main page.

I tried to create a custom post type and create new single file called single-{post-type}, but it gets ignored and the index page gets loaded instead (not even the plain ‘single.php’, which would logically be the next in the hierarchy).

So I want to try another solution, which would be even better – simply create the microsites as pages, apply the new template to them and have them separated from the rest of the pages. It is the last part that I’m having problems with…

Is there a way to separate some pages from the rest ? If so, could someone please point me in the right direction ?