First thing's first: To get to Philly, take New Jersey Transit to Trenton, then transfer to the SEPTA rail system and head to Suburban Station. Roundtrip transportation is about $24.
Arriving at Suburban Station puts you close to the city's Museum District. From 16th and JFK Boulevard, you'll have about a 10-minute walk to the Mutter Museum of The College of Physicians of Philadelphia, located at 19 South 22nd St. Admission is only $8 with your college ID. Just as advertised, the Mutter Museum is "disturbingly informative." Its big-ticket items are the world's largest colon and the "Soap Lady," a woman whose body turned into soap after she died. Currently, there's an exhibit about lead's effects on the body, with fun (or not so fun) facts about how the metal was used before people realized it was toxic. If you pay close attention, there's something to learn everywhere: Be sure to check out - and be disturbed by - the scraps of leather made from human skin.
Obviously, that museum is not for everyone, and if you think it'll spoil your appetite for cheesesteaks (a must), you can opt for one of the many other museums in the area. There's the Rosenbach Museum & Library, with manuscripts from many renowned authors, and the Rodin Museum with a 1925 cast of "The Thinker." Science buffs may also enjoy the Franklin Institute, which features interactive displays and educational programs on science and technology.
Then, of course, there's the Philadelphia Museum of Art, which is only $10 for students and "pay as you wish" on Sundays. I recommend stopping by for two reasons other than the art inside. First, it provides a magnificent view of the Benjamin Franklin Parkway, the flag-lined highway leading up to City Hall. Secondly - and much more importantly - if you come to Philadelphia, you've pretty much got to run up the "Rocky" steps. I took the opportunity to race my friends and strike a victory pose at the top. If you're lucky, an observer will start humming the song for you. We were quickly followed by a group of men in more authentic grey sweatsuits, recreating the same scene. If you're not feeling touristy enough yet, you can get your picture taken at the bottom of the steps with a statue of Rocky himself.
En route to my next destination, I passed by the big historical sites: the Liberty Bell and Independence Hall. I had visited them on past field trips, but you should definitely see them if you have the time and interest. Nearby, at 51 North 12th St., is Reading Terminal Market. It's a bustling, lively place filled with stores to shop for food and artisan merchandise. I loved the energy of the place - the lights, the sounds, the smells. It's sensory overload, but it's lots of fun. Pick up some food if you want, but save room for the next stop.
That would be Jim's Steaks at 400 South St. While Jim's isn't the most famous cheesesteak place in Philadelphia - that honor goes to rivals Pat's or Geno's, depending on whom you ask - it's much more accessible and arguably just as good. I had to wait in an ever-growing line for about half an hour after arriving at 5 p.m., but the wait is always worth it. The cheesesteaks are greasily fantastic. Lots of people go for Cheese Whiz with their steaks, but provolone and American (my recommendation) are also available. Enough people take out that there should be plenty of seating upstairs if you want to sit down to eat.
Otherwise, you can take your cheesesteak on the road for a walk down South Street. My day trip got rained out at this point, but there are plenty of stores, cafes and restaurants to check out in nice weather. South Street is known for its variety, so there should be something for everyone.
From there, you can easily walk to the Market East Station on 12th and Filbert streets and head back to Princeton via Trenton.