If they're interested in software and hardware, robotics is a fun place to start.
Line following, object avoidance, maze solving, all sorts of cool options.
A relatively cheap option would be the micro:bit [1] and a chassis [2]. The micro:bit can run CircuitPython [3] which is an simplified version of Python that can run on embedded systems.
This setup offers lots of room for growth from graphical programming, to circuitpython, and then on to C programming.