Upvote:6
If you want to (or have to) leave the transit lounge of the airport, you do need to be allowed to enter the Schengen area according to the usual rules. There are no exception for passengers in transit or short-term visas on-arrival like there is in some other countries. Depending on your situation and in particular on your citizenship, it might indeed mean that you need a visa. US citizens do not.
Finally, if you are from a country whose citizens do need a Schengen visa, being a US resident or having a US visa does not help (it does exempt you from any visa requirement for airport transit but not for a short visit to the city while in transit).