External screen is a bit hard to use on a train though

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