Here is a link to a list of 42 things to Tweet about: http://socialmediatoday.com/pammoore/471693/42-things-do-twitter-besides-tweet-spam-coupons
If you prefer shorter lists, something you can get fast, here's my list of 5 things to Tweet about:
1. Something interesting you or your organisation is doing, or an event in your lives.
2. A blog post, Facebook page or other post you have created.
3. A reply to an interesting post, mention or retweet that someone else created.
4. Something funny, a quote, a joke, a comment.
5. Something serious, a quote, a saying, a comment.