If C/Cpp is your main focus than why not just programming projects?
If you don't want to stray from the hardware side go for some ARM MCU, there are some nice dev boards out there, use something like Keil (32kb size limit enough to play with) and fiddle with registers and stuff, not relying on plug and play libraries as you probably did with Arduino.
Not sure how you consider just an Arduino inferior to a robot for learning C. Complexity of projects with Arduino is totally upon youself.
I see a robot as an advantage only if you want to experiment with more advanced control algorithms (e.g. nonlinear control ...) but C has very little to do there.