After a youth spent playing with Amigas and getting into all sorts of trouble on the school computer network, I’ve always had a soft spot in my heart for hardware from the 80s and 90s.
The PS/2 synchronous serial clock is easy to interface with ... The 16u2 is normally used as the Arduino Uno’s USB interface. The only external hardware needed is a single 120 ohm resistor.
Although the Arduino Uno can power a host of projects ... you can extend all the basics you’ve gained about C++ programming and serial interfaces with Arduino boards to different microcontrollers.