To-do lists because they make you feel like you did the most efficient thing with your time, you knock out things that you needed to do anyway. Time tracking to make sure you're making the most out of your time. At some point, I was insanely tracking my days by 15-minute intervals, it keeps you on your toes but also made me feel bad if I spent some time daydreaming.
But honestly, and this might be because I have many responsibilities and am a busy person, having nothing on my schedule but still choosing to go outside and see what random adventure life takes me on is my favorite thing to do on a free day. Yes Man Style.