How to End the Day With Peace of Mind

What is the absolute minimum I need to do tomorrow in order to end the day feeling like I haven't completely wasted my time? That's not going to be writing 3,000 words first thing. Instead, perhaps it's just opening the Word document you've been working on and reading through the last couple of paragraphs you wrote. Perhaps it's answering that one email that you've been ignoring or looking up a reference. It should be easy and attainable. Once you've done it, you can carry on and do other things if you like, but that's optional. If you don't do anything else, you still get to finish the day with your peace of mind intact.