Is there an app that's similar to Calendar + Wunderlist?
I will suggest you the Clear App.
Is a minimal to-do app that allows you to create multiple lists and items within that lists. Furthermore, will allow you to set reminders for every item in your lists.

The app is really minimal which help you keep organised. Another feature of the app is that the higher an item or a list is in your app, then the higher the priority is and therefore the colour (first item is dark red, indicating importance and the last one yellow, indicating not that important).

Is also available for iOS (iPhone, iPad), Apple Watch and Mac OS X, and sync over iCloud to keep everything synchronized.

Is not free though, but for me is my main to-do organiser app for a long time, so for me, worth it's money.