There are many fantastic MIDI projects on the Adafruit Learning System for you to get further inspiration from. These projects are full builds compared to the examples in this guide, so you can get some ideas for enclosures, hardware and more advanced code.
This project uses 16 light-up arcade buttons to send MIDI note messages out. Additionally, it has a 5-way joystick and a screen to change the MIDI note numbers assigned to each button on the go.
This project uses the Trellis M4 to have 32 button inputs for sending MIDI out. Additionally, it uses the built-in accelerometer to control MIDI parameters by panning and tilting the board.
This project sends MIDI out to control solenoid motors. The solenoids are setup to hit percussive objects to create a robotic percussion instrument.
This project sends MIDI note sequences out. There are parameters for changing the note pattern, key and note value. Additionally, there is a slider that affects the BPM, or the speed, at which notes are sent out.
This project uses BLE MIDI to modify a classic Nintendo Power Glove to be a wireless MIDI controller. Flex sensors in the fingers are used to affect the values of the MIDI messages.