Is there any simple way to do this?
I have used NewsPublisher and it can "just... make an area editable" - there are some limitations but I think one could work around them if the page design/layout was correct. I added NewsPublisher to an existing site some pages were fine others did not work - I had no need to work around the problems so I have not.
I have not checked out UpFront, but unless you're up to fixing things I would be wary of a '0.1 alpha' last updated just over one year ago!
I might still check it out, just be wary and prepared for some coding.
Is there any simple way to do this?
What is this? Exactly.
But the answer is probably no.
You could also check out Aloha Editor.
I just did a search and found the same person/people that created UpFront also did this:
"AlohaX - a basic front end editor using Aloha for MODX Revolution. It's not well tested, probably very unstable but ready for some feedback and hopefully it get's forked on github

"
See:
http://alohax.butter.com.hk/
and:
https://github.com/digitalbutter/AlohaX
Let us know what you end up doing.