St. Mark's Church in-the-Bowery, which is New York's oldest site of continuous religious practice (over 350 years!), has a secret literally buried underneath it—Peter Stuyvesant's remains.
The site of the current church used to be where the Stuyvesant's family chapel was located. When Stuyvesant died in 1672, he was buried underneath it. The land was sold to the Episcopal church in 1793, and by 1795, the cornerstone of the current church was laid.
Fun fact: Alexander Hamilton, who's buried down at Trinity Church, helped incorporate St. Mark’s as the first Episcopal parish independent of Trinity Church in the United States.
You can see the grave in the church's graveyard. You can also see a stained glass window in the church that bears his likeness.
In addition to sermons, the church today hosts a variety of secular events, including classical-music concerts; organizations like the Poetry Project (which holds its annual New Year's Day event here) and Danspace Project also call the space home.