Every visitor to Philadelphia - whether one is there for a day, a week or a month - must visit Independence National Historical Park. Not only is the park a beautiful part of the city, it reminds all visitors of how and why our country became a republic. The sites are free and almost all of them are open daily from approximately 9-5 (tours of Independence Hall require a timed ticket during peak season March-Oct and Declaration House is only open in the morning). Bishop White House and the Todd House (home of Dolley Todd Madison before she met James) are shown by tour - the tour is free, but timed and arranged at a counter in the Visitor Center.
The message is this: before your visit, check out the maps and site descriptions at www.nps.gov/inde When you arrive in Philly, hit the Visitor Center to be sure you have the correct times/days open so you can efficiently plan your tour of the park.
I don't think that hiring a tour guide is necessary - the rangers are so kind and willing to answer any questions. The maps are good and the pamphlets are informative. If you are in good shape and a real "go-getter", you will be able to see just about everything in one day. However, I recommend two days to explore the park as there is a lot of information to take in.
Please note: Some sites in and around the park are not open on Mondays and/or Tuesdays. Do not rely on guidebooks - the descriptions seem to be aimed at weekend visitors.