As has already been mentioned, the weekend of the Papal visit is a good time not to be in Philly.
SEPTA transit service will be greatly modified, and hotel rates, which are usually far less expensive that corrider cities like Boston, D.C., or New York, will soar, which is also the by product of Center City's hotel room count being lower than the other cities mentioned.
SEPTA offers seniors a grear fare on city transit (as already mentioned) , and the non-senior travelling with you can get a SEPTA Independence Day Pass
for $8.00 per day, and although it is called a pass, the limit is 8 trips per day.
As for the sights, stop by the large visitors center on Market Street by Independence Hall, it is a combined city/historic district center, and stop there
BEFORE visting any paid sites, as they have many discounts available. There is also a pass that lets you visit a number of paid sites at a discount.
http://www.phlvisitorcenter.com/
One site that is off the radar screen, and I highly recomment is the ride to the top of City Hall tower, and is offered on weekdays only.
The Reading Terminal market is a must for lunch.
Lastly when touring through Center City, it is always good to be aware of your surrondings. Center City is by and large very safe, but Philly as a whole remians a very poor city, and sometimes it brings out those who like to prey on unsecpecting tourists.
Ken
Elfreth's Alley (the nations oldest residential street, and is used to this day) is a great site, and an easy walk from the Betsy Ross House. I lived there for a time, and the residnets are used to all the tourists.