It’s a no brainer. By definition it means something that requires little thought.
This is a day that is about not thinking hard and resting your brain. Things that are a no brainer are things that correlate to things that are common sense/obvious or things that have become habitual.
This day is full of endless tips of random no brainers, this list is non exhaustive, but here are a few random tips to start your day.
Organize and declutter, make sure everything you own has a home
Clean up i.e take out trash, clean up spills, put things away
Unplug or turn off things that are hazardous
Keep track of what you spend, to create healthy financial habits
Be aware of what you eat daily, to create healthy eating habits
This day is a good way to start or change certain habits, and also a way to relax if you overthink or do something that requires a lot of specific types of thinking.