Dear Notturno:
The forementioned Sonata K 545 is a great choice, although you might look at the third movement because it's tricky. Also the Sonata K 282 is not that difficult.
Anyway, I have to disagree with Kelly: the Fantasia K 397 is a bit harder than 5th. In general, it's not a difficult piece, but its cadenzas requires really fast and controlled scales. Furthermore, its interpretation lies upon several contrasting moods, and hence is not easy to render.
If you are not concerned with playing a complete work, there are sonata movements that you could think about: 331/3 or 280/2, just to quote a few examples.
By the way, Mozart composed some variation sets. Despite the fact that some have virtuosistic features, you shall find one that meet your possibilities. Again, if you're not talking about recitals or something in this way, you can cut off a variation that you feel unconfortable to play.
Hope it helps. Best wishes!