Stupid question – altimeters.
Some altimeters have one window for inches of mercury and one for millibars. There are conversion charts, but you would only want to do that for a limited period of time.
On the part about metric being easier. There are only a few countries that use metric altimeters. With the advent of RVSM the 1000' spacing has become a contentious issue. Some countries have come up with a system for issuing clearances in metric while actually flying in feet. While other countries have just decided to issue clearances in feet.