The PI is always a good place to start, but they’re not cheap anymore. You can still do some useful things at the command line (not sure how fun, but a great education), python is there and very accessible. - get a camera and you could do some great things with open CV. Not sure what packages are out there though. Think you’d just have to follow some web tutorials.
As an alternative, have you considered an Arduino kit? Lots of great projects, all very well documented. Playing with LEDs, sensors, motors etc may keep their attention longer than a bash prompt.