VentureForth--just a couple of other tips:
If you get off at 8th and Market, to get to the historic district, walk two blocks to 6th and Market. Walking down 6th street, you will be walking along the historic area. Also, City Hall does timed tours and a security check if you want to go to the top, and the Liberty Bell and Independence Hall are federal, so they do a security check also. If you are pressed for time, it might be best to choose just one of the three. (I would suggest Independence Hall--you can pretty much see the Philly skyline from Amtrak, and if you walk down 6th Street, you can see the Liberty Bell through the glass at the end of its building.)
If you are coming into 30th Street on Amtrak, the Travelers Aid booth can give you a nice map of the city. I'm sorry, I am not familiar with the trolleys (just SEPTA and the buses), but the Independence Pass suggested elsewhere is wonderful and could help save time. (You could take it back along Market Street to City Hall, for example, and save the time that walk would take.) Also, if you end up in Market East station for any reason, remember that it is now called Jefferson Station.
Finally, one more suggestion: come back again and stay longer! Philly is a nice day trip from where I live, and I never get tired of it!