Well, it depends on how much time you have and the level of your skill. In general, if you have the skills and just searching for a good topic then the solution is quite easy.
Just look up on what you are interested in and go for it. For exmaple, some programmers prefer developing games like: Aliens Invaders, Strategy games, arcade games... etc. Others like more solid projects like an accounting system or some sort of simulator. Solving and implementing math problems might be a option here. Applications involving Physics, Chemistry, probability also can fit in many different forms. Thus, see in which area your interest lies and then head your attention towards it.

Good Luck :D