Maybe the function that changes degrees F to degrees C? Maybe the function that changes Kilometers to Miles? Maybe cut and paste the last month of data from, say, Google Inc. stock or equity value for each day of the month in Dec? Then compare that to a "wild" sort of penny stock, so students can see a "stable" function vs. an "unstable" or erratic penny stock equity value that changes much more drastically? Just draw a F(x) "box" and talk about how the Market opens in the morning and closes at the end of the day (input price, output price--however this may be confusing if the stock changes in overnight trades). If you find nice stock charts that are approximated by simple linear or second order polynomials, that might be cool.
There is always simple projectile motion, if you give them the equation and talk about angle of shot, initial velocity and vertex or highest point reached (maybe too complicated?). I bet there is a youtube video on this, and maybe an applet to play with by changing the initial values or constants.
Maybe have students take data in the LUNCH LINE--say, cost of meal vs. a "popular vote" on the "most enjoyable" OR the "most nutritious" lunch (main course, two sides, and drink)--and plot these two functions using the cost as the ordinate.