SynthBox and MIDI_Wand are two instruments I built using the teensy microprocessor platform and the wonderful audio library that goes with it. SynthBox uses the teensy 4.1, which can power an external MIDI keyboard using a built-in battery and the USB host library. In addition to the MIDI keyboard, it can be played using OSC over a WiFi network using the added ATWINC1500 chip.
MIDI_Wand is a MIDI controller (and FM synth) that uses an IMU sensor (pitch/roll/yaw) to generate melodies. Buttons add simple control of the melodies and allow you to change the type of scale.