I know this is a radical statement but it's true!
You don't need a day planner because you already have one that you carry with you 24/7/365....YOUR SMARTPHONE.
Most don't like to part with pen and paper but what we don't realize is our phone can actually act as a personal assistant.
You can braindump everything on your mind in a regular notebook (I love the $.88 composition notebooks) and organize all your to-dos in your reminders app and your calendar.
Your phone will then remind you off all the things you need to do with margin in your time. You don't even have to think about it because your phone is doing it for you!
Bonus if you sync it with your Alexa/Google Home/Apple Home devices. They will read your daily schedule off to you. Have a multi-million dollar CEO style meeting with yourself every morning over your coffee if you like .