A little trick I like: You create your own custom language in Notepad++:

1. Go tab Language → User Defined Language
2. Create new… button
3. Name it whatever you want, txtRegions or whatever
4. In the “Folding in code 1 style” area put your favorite Open and Closing symbols. For example I used {{ and }}
5. Close the dialog
6. For your todo file, now go up again to Language → Select your custom language at the bottom
7. Try it out, you can now make your own regions in your .txt file

 

Usually Notepad++ remembers your custom language for your file. If it should ever forget to use it (and falls back to default .txt behavior) just select your language again. Have fun! :)