Personal growth

read a book or article on self-development;

have a meditation session or learn how to breathe properly;

take an online course in a foreign language, speed reading or something else;

do a physical warm-up that does not require a lot of space and extra activity: stand in the bar, do push-ups, squat, shake the press;

keep a diary, write down a plan for tomorrow in it, analyze your productivity today. Make it a rule to do this daily.