OneNote - use tags. I mostly use the checkbox and question tags. You can pull up every unchecked checkbox in a document or review everything labeled a question. Great for finding things you forgot to finish. I borrowed from Wiki and made an stub tag for things I want to fill out later after I'm done brainstorming.
OneNote - put double brackets around a word or phrase to create and link to a page with that name. Great wiki functionality.
Keep - Pin important actionable items. Make a widget and put it on your homescreen. Set it up so that at the end of a productive day your todo list is empty.
Keep - Use location based reminders. This feature alone has saved me many trips to the hardware store.
Keep - Use hashtags to tag items. A great way to jot down ideas and review sets of them at a later date.