The X project

I've been for almost two weeks and I continue, looking for something to do with the knowledge I have learnt. I need something. But I can't think of anything!
It looks easy, but nothing comes. You'll see:

A thing suitable to be built has to meet these two requirements. (Obviously, I have to be capable of making it):

1.  It has to be cheap.
2. It has to be useful.

Easy. I'm blocked...