Ruminating (actually, procrastinating, who am I kidding?) on what makes a day productive for me, I came up with a metric that rings true so far.
It's time spent creating vs. time spent consuming.
Creating is making new things or making things better. Or at least trying. Consuming is usually doing things you shouldn't be doing: reading Secret, playing that game on YouTube because you're too busy to actually play it, picking up your phone every minute to check for notifications.
Now, not all consuming is bad. More often than not, kicking back and reading a book for a while or daydreaming does more good than harm. You can't spend all your active time creating, or you'd run out of creative juice. You need time to rest and recharge.
Still, creating creates (ha!) momentum. I've noticed that if I start my day on a creative note, I get more done. Rarely what we need to do is to consume something, rather it's something we should create: write emails, code a program, present our ideas to other people.
Consuming works the same. If you eat (consume) a cake instead of doing a morning pushup set (creating a better body), you're much more likely to slump on a sofa and watch (consume) another episode of people in fantasy setting killing each other right and left than draft (create) a new blog post.
This reminds me of the concept of stability in physics.
Stability is the ability of a body to restore its balance after a disturbance (change in position or orientation). The quality of the stability is determined by how large a disturbance the body can withstand before the balance is lost. A body that is precariously balanced can withstand only a small disturbance and so has low stability. A body that is solidly balanced can be disturbed greatly and so has high stability.
Consuming is settling on the bottom of a pit: you have nowhere to fall. Stable. Creating is rope walking: at first, every misstep sends you tumbling into the pit. As you learn, even in headwind and with the rope thrashing around, you remain stable atop. When you do fall, you're back on the rope in no time.
So, time spent creating vs. time spent consuming. At the end of the day, concentrate for a few seconds and in your heart you'll know at once whether you've spent enough time creating.