Gakusei is an online (and real-time) tool to study and test your Japanese knowledge. It can generate thousands of different sentences by rearranging words and sentences, although its vocabulary is a bit limited right now.
Try it online
On your browser: learn.fragments.me
There's also an android app wrapper for your pleasure.
Or run it locally
(if you don't have meteor, head up here https://www.meteor.com/)