This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
Set yourself the challenge of making a game such as tic-tac-toe, quarto or even chess. Make your first task to be able to play the game through the console. Then try adding a GUI. You should also add multiplayer through LAN. Finally you can attempt writing a computer AI for the game.
You can learn a lot of different things from such a challenge. The project usually also becomes big enough that you have to learn how to structure it well.