Gather a bunch of self-propelled vehicles. They can be wind-up or ones you have to pull back and release.
Use masking tape to make a starting line and a finish line 1 meter apart.
Wind up (or pull back) the vehicle and place it at the starting line.
Let go and start a stopwatch. Time how long it takes for the vehicle to reach the finish line. Record the time and determine the speed the vehicle travelled.
The obvious unit to use in this activity is m/s, but you may want to challenge some of your students to covert it to other units: m/h, cm/s, etc. Or, really test them by having them convert to miles/hour!