Learnt some C programming - now what?

Not sure if this is allowed here: I learnt for my university exam this semester some C programming (if, else, loops, functions, arrays, pointers, simple algorithms, linked list, other basic stuff).

But now I am wondering where to go from this? I "know" how to do stuff in C, but I lack the knowledge to do actual cool things. Would want to hone my skills as a programmer and start to tackle some intermediate projects. Maybe go more low level, or do something not too crazy with a gui, like moving some boxes around (a tetris game maybe?). I want to do something cool, as the simple pocket calculators start to get boring.

Where and how can I find knowledge? (I read Kochan - Programming in C and will probably reread it again)

Edit: This was an awesome community support. Thank you so much.