At my Manhattan College Summer Institute, participants can earn graduate credit by submitting a "final project." As one focus of the course is the development and use of quantitative demonstrations, an option for the final project is to create a couple quantitative demonstrations for use in participants' classes.
Alex Tisch, a colleague of mine starting this year, took this institute and submitted a clever but involved demonstration he had brainstormed. Before I give the details, I ask you to BE CAREFUL with this one -- it involves a live, full-sized, moving pickup truck. I generally scoff at the dire warnings that textbooks present with their lab ideas ("Drop a tennis ball and time its descent. But WEAR GOGGLES!!! And a cup!")... but don't do this particular experiment unless you're sure you can accomplish it safely. If you're worried, have the class watch while a few faculty members carry it out.
Alex envisions his pickup truck in neutral on a level surface. One person is in the driver's seat, just to keep the truck going straight. One person sits in the back with a turkey baster full of paint; maybe another person is back there with a stopwatch.
A big strong person pushes the resting truck, trying to apply a relatively constant force. Once he starts pushing, the painter drops one dollop of paint off of truck every second.
The class can now measure the distance from the starting point to each dollop of paint on the road. From this data, a position-time graph can be made; analysis of this graph can lead to a calculation of the acceleration of the truck. Knowing the mass of the truck + occupants, the average force applied by the pusher can be calculated using Newton's second law.
Neat, but not awesome yet. Alex's big brainstorm was to MEASURE the force of the pusher while he's pushing. He envisioned a bathroom scale with a nearby observer to record the force every second. I suggested a force plate. (A force plate is essentially a bathroom scale that connects to Vernier or Pasco data collection softward, so can make a live graph of force vs. time.) Vernier's force plate even comes with handled attachments that make pushing pretty easy.
The force calculated from Fnet = ma can be compared to the force measured directly from the force plate. Awesome.
(Picture from Leatherneck magazine.)