Wooden blocks and wheels that's all we need to explore speed. We've been making ramps of different slope steepness and racing cars down them timing them with a stop watch. Everyone enjoyed racing their cars zooming down the big slopes. Afterwards we also experimented adding material to vary the friction on the slope. "The car's not moving because the ramp is fluffy" said one of the children. Learning how to make predictions and come up with explanations are key foundations in science.
"The most exciting phrase to hear in science..is not 'Eureka!' but 'That's funny!.."