Simple desktop that works and isn't crazy: Linux mint with cinnamon desktop. Just use it, I promise you won't regret it.

I don't use that, because I don't need the simplest thing, I'm a more complicated person. But my wife does, and it's amazing. I set it up for her. I tried out a handful of desktop environments the other day to see which one is the easiest to use, cinnamon is the only one I tried that I'd recommend, which surprised me. KDE just gets in your way every chance it gets, others just deviate too much from the windows concept of a desktop for someone who isn't into Linux to start using, and the rest have quirky behaviors and bugs that just make life harder.

As far as the distro, you'll hear mint a lot, it's what Ubuntu used to be back when it was the easiest to use distro. I'll always recommend it to people that just need a desktop that works. Linux Mint with Cinnamon Desktop, your parents won't even notice except that they don't have forced updates and login to outlook and all the terrible cruft, they'll just have a computer that works for them again.