Hi guys, and welcome to Piriform.
I think the easiest way to get back to the default settings, is to go into CCleaners Program Files folder (C:\Program Files\CCleaner), and delete the "ccleaner.ini" file, which keeps your various configurations.
Basically, it's all checkboxes in the Advanced section cleared, and everything else checked apart from the obvious ones you know you personally want unchecked.
Deleting that file, would also lose any "includes" or "excludes" you have, or any cookies kept. In that case, rename the CCleaner file to "ccleaner_mod.ini" for example, re-launch CCleaner, check out the settings, maybe take a screenshot.
Change "ccleaner_mod.ini" back to "ccleaner.ini", and then redo the checkboxes based on your screenshot.
In the registry section it's all boxes checked, although you don't have to use that section if you aren't sure. If you do use it, please make sure you follow the CCleaner dialogue, and make a backup.
And yes absolutely. A "default" button would be helpful to many people, including me after trying to accurately answer this one.
Also check out the Piriform Docs site for guides to all Piriform software. Link in my signature.