I'd suggest skipping the Independence Hall and Liberty Bell, unless you really really feel the need. Ridiculous security and difficulty of getting tickets to the former rather ruin the experience.
You can get an early start Friday by attending the First Fridays event. Galleries stay open, many offer free food and wine.
As with much of Philadelphia, the Art Museum seems to be surprisingly underrated. It has an impressively large collection and of course you can run up the steps and pretend you're Rocky. Rodin museum is nearby.
If you get a chance, pop into the Reading Terminal Market which is near the hotel. There's also the Italian Market which is a bit farther away.
For general soaking up the city, Rittenhouse Square is probably one of the best urban parks in the country. There's lots of restaurants/retail around there for good strolling.
Alternatively, there's the Delaware Avenue end of South Street, and the general Old City vicinity.
The Marriott is close to Chinatown, providing Sunday morning Dim Sum opportunities.
