Well, it sounds like you've already started! But if you want to add more, just tell them they're doing so well that "it's time to take it to the next level." Then introduce the ScoreBoard. I started much like you. In fact, I think it might be best to go slower - implement as little or as much as you like, but don't try to do it all at once. I'm in my second full year of using WBT and still adding components (this year I've developed PowerPix for my 9th grade Science content).
In short, it's never "too late!"
- Jen