Mathalicious has lots of great activities, but this one might be my favorite.
Here is the link: http://www.mathalicious.com/lessons/domino-effect
All I do as the teacher is give them this prompt:
Domino's pizza is delicious. The company is tech savvy, you can order online and even track your pizza delivery. The website is great, but not transparent. Domino's does not tell you how much the component pieces cost; they only tell you an item's final price after you build it. Your job is to find the base price of a pizza and find the cost per additional topping. You need to have enough information to do a 2 minute presentation to the class.
I don't grade the presentations, but look for slope, y-intercept, a graph, an equation, and an explanation of the graph. It is a fun 30 minute project that fits at the end of the unit.