You don't mention what type of applications you're interested in, Web apps or something else.

The most fun programming i ever did was for my Digital Image Processing class, mainly because the output was so visual. You might try something visual, graphics(a game perhaps), image processing(non photo realistic rendering of a pic).... you get the picture :-)

Edit: you might even do some visualization of data, using processing.js for example. This would not require much time and you can even share them as a web app.