The simplest project that covers the basics of motors and controls is a self-balancing robot. 2 Motors, an h-bridge, a battery, an IMU, and a microcontroller.

A step above that I would say some kind of wheeled robot with more complex motors that have encoders, and ROS if you want to mess around with autonomy. If you have the budget for it, a stereo camera or $400 time-of-flight RGBD camera can get you pretty much full autonomy (if you configure all the ros nodes for it).

If you are into robot arms, LeRobot has a desktop robot arm kit that uses neural networks to control a robot arm with a camera for seeing the world.

If you want to learn the theory, I really like Probabilistic Robotics by Sebastian Thrun, and Modern Robotics by Lynch and Park. Those cover pretty much all of the foundational concepts for autonomous robots.

The most fun thing I think currently are drones. I have been designing and building a fixed-wing drone with autonomy and FPV capabilities. Started from zero information and just learned as I went. Plus flying drones is more fun than driving robots IMO.