We created Motion ( https://usemotion.io ) for exact this purpose. It's a calendar with to-dos built in that helps you protect your time by letting you blocking it off. To-dos take time just like meetings/events, and having them on the calendar is a productivity game changer.

Probably the most powerful productivity boost I've experience is that when I use Motion my schedule isn't interrupted by all the less impactful work that pops up because my time is already blocked off for the day.

If you're interested in checking it out, mention you came from needgap in the sign up form, and I'll hook you up with an invite: https://usemotion.io.