I assume you're going to place health/office related adverts in the emails, and Like/+1 links?
You could perhaps add a link that people can click once they've completed a task, and maintain a running score for them or something? Turn it into a game.
There already is a Like link there...but that needs a little tinkering to get it to behave like I envisioned. Will need to add +1 since that's getting popular. Another one I've been contemplating is to add Twitter link as well.
I think your idea of turning this into a game like experience is brilliant. One thing I've noticed with emails of this nature is that after a while you start to ignore them. Letting readers keep score of the reps/tasks completed would hopefully entice them to keep opening the emails and, more importantly, keep them exercising.
And thanks for signing up!