You can find a farmers market in the city almost every day of the week; like the mall food court, the actual food won't be free, but you can totally fill up on free samples.
Philly has a ton of murals (over 3,600), so take a free tour and spread the social love for the Philly arts.
This free ride around the city is technically for people going to/from SugarHouse, and you've got to be over 21, but if you've gambled there before, they probably owe you some money anyways.
On the first Friday of each month, the city's art galleries stay open late for showings, often with some free booze and snacks.
Did you know Philly's favorite son Ben Franklin started America's first fire department, the Union Fire Company, in Philadelphia in 1736? Head to the Fireman's Hall Museum in Old City to learn other super-interesting facts like this -- and see really cool antique fire trucks!