Hi; Sounds weird but I will suggest one :D If you are looking for a general purpose Circuit then : I advice you to make a PCB where you put some training components for embedded systems programming like : Push buttons ; Potentiometers ; 7-segments display Some LEDs Some motor drivers like l298 IC And all powered by an ATmega328p TQFP package MCU that you can program through a USB to serial chip like the famous CH340 or an FTDI chip Text me here if you need further guidance