Have you tried a team challenge on textivate?
You can do this during class time or between classes, or from one week to the next. (Just make sure to set an appropriate end time for the challenge.)
A simple team game, played ideally with a fairly short text.
In the example here (see image above and live example below) I have chosen to affect 1 in 3 spaces in the text (i.e. 2 in 3 spaces between words are already present) and students have 48 more spaces to find.
Divide the class into 2 teams and select a member from each team in turn to play.
The rules: