Whether you are a budding writer or an experienced pro, I recommend the Your Messages exercise that's going on at the moment. If you pop along to that site, each day in November there's a prompt, a short piece of text. Your mission, should you choose to accept it, is to write something in response to the prompt, exactly 30 or 300 words long.
When I heard about this, I wasn't entirely sure about it, but once I had a go I was hooked. It doesn't have to take long at all, and some of the results are great to read. Give it a try. (But don't read the others until you have submitted your own).