If you haven't taken a look at Google's GMail Sidebar you need to.

Along the right side of your GMail window you will see a Google Calendar icon, Google Keep icon and a Google Tasks icon.  Click on any of these icons and Google opens a side window that displays your calendar, notes and tasks.  Flip back and forth between these windows by clicking the appropriate icon.  

Using these three Google Apps alongside Google Mail makes a single Chrome Browser window into a daily planner.  Move back and forth between your to do lists, calendar appointments and notes without having to change windows and lose focus on an email that you are dealing with.