The historic city of Philadelphia is just across the Delaware River from New Jersey and is worth a trip for many reasons, not least of which is putting yourself in Mattie's world of the 1793 Yellow Fever epidemic as depicted in Laurie Halse Anderson's wonderful young adult historical novel Fever, 1793.
Start where Mattie says her family's coffeehouse was located (coffee cup icon), at the corner of 7th Street and Market Street (then called High Street). Now there is a Dunkin' Donuts on one of these corners where Anderson may very well have imagined the coffeehouse to have been located.
Then head two blocks east on Market Street (towards the Delaware River). Between 6th and 5th Streets, you'll find the sight of President Washington's residence in Philadelphia (yellow house icon). The house was demolished in the 1800s, but recent archeological work has been done on the site and you can see down into the old basement.
Keep heading east until you get to Third Street. In 1793, this whole route would have been a bustling open-air market. Mattie was forbidden by her mother to shop any closer to the waterfront than Third Street (indicated on map with red line). It was feared that the fever would be caught more easily there. Make a left on Third Street and then a right onto Church Street to find Christ Church (purple balloon icon). Look up. The steeple was added in 1754 and for 50 years after this addition Christ Church held the distinction of being the tallest building in North America. Mattie would have heard the bells from this church ring every time a person died of Yellow Fever.
After visiting this historic church and its surrounding grave yard, head back toward Third Street and Market Street. Keep walking south on Third until you get to Chestnut Street. Make a right. Between Fifth Street and Sixth Street, you will find Congress Hall (flag icon), where Congress met from 1790 to 1800. Mattie could see this building from her bedroom window.
There are some other locations on the map (like Bush Hill Hospital and the town where Mattie and her grandfather were headed). They are beyond walking distance, but it may give you an idea of how far Mattie was from home.
View Mattie's World, Fever 1793 in a larger map