The weather forecast for Sunday looks good, so I've scheduled our meetup for Washington Square Park again, between 6th & 7th Streets on Walnut in Olde City. Although it'll be in the low 80s, there's plenty of shaded areas in the park. If you don't want to sit on the grass, just bring a folding chair or picnic blanket to sit on. You can also bring any type of food or drink you want, although I don't think alcohol is allowed in the park.
(If the weather changes, our fallback spot will be Café Walnut, which is right off the square at 703 Walnut Street.)
The park is fairly easy to get to if you're using public transit. With SEPTA, take the Market-Frankford Line & get off at the 5th Street Station (corner of 5th & Market), and walk 2 blocks south on 5th and then turn right on Walnut Street and walk 1 block west. With PATCO, just get off at the 9th-10th & Locust stop and walk 3 blocks east. For those who are driving, parking in the neighborhood can be tough to find. If you can't find a spot on the street, I'd suggest parking in the Washington Square parking deck at 249 S 6th Street which is just a half block away.
