Do they know how to keyboard well? If not, I'd start with typing software so they develop good habits. There are plenty of options that are fun. I'd personally have no trouble making that mandatory for 10 minutes a day during the summer.

You may also want to take a look at the book Games for Writing by Peggy Kaye. It has a lot of fun ideas. http://www.amazon.com/Games-Writing-Playful-Child-Learn/dp/0374524270