This is a small modification of the Arduino Built-in Example "Use Multiple Serial Ports on the Arduino Mega". The UF2 file was compiled for the Adafruit SAMD21 QT PY, but the INO file should apply to many other MCUs.
Why buy a USB-to-serial converter board when you can be a hardware badass and re-use an MCU board with a USB port? Sure, the USB-to-serial converter boards (FTDI, CP2102N, CH340) are convenient and configurable, but everyone else uses them and you want to impress your friends.
I have made a couple of UF2 files with fixed baud rates, which can be drag-n-dropped onto a SAMD21 QTPY board.
Enjoy.