There is a table top sitting across the 2 student carts. You can walk on this "frictionless platform" and discuss center of mass. As you walk forward, the platform rolls backward, then walk backward, platform rolls forward and you end up back in original position. Can discuss canoe in water problem.
There is a meter stick on the cart which can be balanced on a rod sticking out from the frame. You and a student can "weigh" a penny. Balance the meter stick, then place penny on 0 cm. Place 20 g mass around 56 cm to achieve balance. Should get something like 2.4 g. Note: pennies before 1990 weigh slightly more (they are solid copper).