What are you interested in? Do you have any hobbies? What techniques or technologies would you like to learn? Look at all the different kinds of apps on the web, does any of them strike your interest? Answer those questions and you will have a good idea of what you might want to build.
a) store app with merchandise and shopping cart, using relational database (Amazon, eBay, Target, others)
b) social network app, using nosql database (FaceBook, Twitter, etc)
c) information app, mostly to store information you are interested in, such as your DVD or CD collections, or information about your hobby