Try creating an image editor. Nothing fancy, just open an image file and allow the user to do basic editing -- cropping, grayscale, rotate left, rotate right, undo, redo, zoom. Make sure you avoid memory leaks and file locks on the images. Don't worry if you don't know much about images, you can pick that up along the way and push yourself to become better. Take your pick of using WinForms or WPF. You'll need to be familiar with at least one GUI library (I'd pick WPF, since your skills would be transferable to Silverlight, but it does have a steep learning curve. Then again, you are doing this to learn.)