Challenging web application to improve web development skill set

[sys_argv]

I want to build a web application that will improve my overall web development skill set and that may possibly be put up on github and used in my portfolio. I have some experience in web development using Python(Django) and PHP. Apart from building a CMS, what kind of web application is technically challenging as well as suitably impressive?

[ainiriand]

One good step forward would be building a web application that crunches a lot of data from different sources and presents it in an original and practical way. For example, something related to flights and hotels and airbnb.