Maths: Compare the distances different bikes (wheels) travel for one pedal push. Make sure your test is fair. Plot the results in bar charts and graphs. Many children should be able to make predictions and form a fairly precise hypothesis. Make a collection of bicycle tyres. Investigate radius, diameter and circumference.
Science: Use an investigation of the cycling process to illustrate streamlining. Why do Olympic cyclists wear the types of helmets they do? Why do they go as far as shaving their legs?