Featherboard M0 Basic Proto
Featherboard M0 Basic Proto is very simple design and easy to use ARM M0 based board.
Arduino IDE supports this board and most of the sample code works with it.
The great feature is that it has a battery connector and it can be used as portable devices without having extra board like other MCU boards.
The MCU is actually same as Arduino M0/Zero which is ATSAMD21G18 48MHz, but the board is much smaller. The technical details are available here.
Featherboard has other versions with WiFi, Bluetooth, and so on, so depending on what you would like to do, you can get the one with those features.
Follow the setup instruction here and here.
How it works is a little bit different from the Arduino, but once it set up, it would be very easy to switch between Arduino and Featherboard.
Once you install the necessary software, it generates most of the source files under your user folder which is \AppData\Local\Arduino15\packages.
If you prefer not to use the predefined APIs, you can refer to these files and generate your own functions.