Nova Launcher
Edge Gestures as an alternative to android gestures
Quick Cursor for easy one handed usage
Panels as a sidebar based quick launch / app switcher
AccuWeather for weather and minute by minute forecast
Google Fit for basic quiet step counting - can also measure your heart rate with the camera
Outdoor Active or AllTrails to show you walking routes
Street Complete to contribute to open street map in a much friendlier way
Spark for multi account email
External screen is a bit hard to use on a train though