Logo is a great tool to use to learn visually about basic shapes and their properties (circles, polygons, conic sections, spirals, various sorts of roulettes, ...), logic, graphs of data and functions, fractals, cellular automata, synthetic geometry including projective geometry, transformation geometry and symmetry, basic number theory, complex arithmetic, polar coordinates, trigonometry, the logarithm/exponential functions, matrix algebra, linear regression and other statistical tools (Kalman filters!), optimization, basic probability, dynamical systems, lattices, group theory, graph theory, vector calculus, mechanics, optics, electric circuits, special relativity, 3d vector graphics, searching and sorting, trees, mazes, robot planning, game AI, ....
Or really anything you want, though after several years of experience students might find other tools more convenient for some types of modeling.
You can do some cool stuff with LOGO like create state machines, implement sorting algorithms, and create simple games.