In general, the best ideas for learning a language would be by designing a game. Everyone likes to play games and thus you could ask for a lot of feedback, no matter how simple your game is. Of course, a Space Invaders clone could be a bit complex but if you have some experience, I estimate that you could do this within four weeks.
When I was learning to program, I just created a simple shooter game where a gun at the right side of the screen had to be aimed at a moving target on the left, simply by pressing the UP and DOWN keys. Pressing Spacebar would fire the gun and with a bit of luck, you'd hit the target. The target was smart, though, and would dodge the line where the gun was located, moving just randomly while it wasn't at the same height.
I wished I still had the code for that. I wrote it 25 years ago, in Turbo Pascal on MS-DOS 3.3, in graphics mode.