Tiny MCU with the same footprint as the XIAO
The Adafruit QT PY is built off an ARM M0 core just like the XIAO. The pinout is compatible with the XIAO and most Arduino code will run without modification.
The main difference is that the QT PY has a built in RGB LED, a reset button, and a spot for a 2MB SPI flash chip. On the XIAO you have to short two tiny pads to reset it.
The optional SPI chip makes this a much more capable CircuitPython device. The 8 pin SOIC chip is available at Digikey GD25Q16CTIGR.
The QT PY is currently $6 from Adafruit or Digikey. It is listed as a "Limited Time" price.
The XIAO (left) has a metal can covering the components. The QT PY (right) has an additional connector for I2C devices.
The bottom of the QT PY is where the optional 2MB SPI flash chip is soldered. I soldered diodes legs to make them socketable.