I think your issue is well taken, within the confines of the OP.
It would require very precise measurement to tell the difference in fall time due to weight difference.
The methods I said would work were those for which even a possibly imperceptible difference occurs.
Drop the coins in an elevator shaft filled with water, tho, and the difference is more easily seen.
I said a method worked when there was a direct effect of density on the proposed observation.
I also postulated a measurement apparatus that had sufficient precision to detect it.
I said a method would not work when there was no direct effect by density on the proposed observation.
For example, making a pendulum and observing the period. Only the length of the string matters there.
In your cycling experience, I'm sure the drag at 70 mph is noticeable. And for your safety I hope you wear a helmet.
For competitive purposes cycling helmets [and body positions] are studied carefully for air resistance.
Electronic timers are sufficiently precise to measure the effect on arrival time when those matters are ignored, even for lesser speeds.
But the OP did not place such sensitive instruments at our disposal.