A no-brainer BYOB is always Italian, and there is no better place to go then the Italian Market area. May I suggest August BYOB? August is a comfy little trattoria located on South 13th street, only open for dinner and only Wednesday - Sunday.
In a place like this, I always suggest that you go crazy on the appetizers and get an entree to split. I'd go for the Roasted Red Peppers, Spicy Sausage and Beet Salad for an app, then split the Farfalle Pasta with crab meat, sun dried tomatoes and porcini mushrooms in a light cream sauce. Its the perfect dish to split, the cream sauce dishes can be heavy but not when you're sharing it... it's just enough.
Italian food and red wine practically go hand in hand... so if you're going to go red I'd go all the way and get a nice Chianti. I suggest a mid priced bottle of La Castellina Chianti. It's a bit on the sweet floral side, however the spice really comes through on your pallet and will pop once paired with the roasted red pepper app and the creamy pasta dish.
If Italian isn't your style, no need to worry. Philly's number one tourism site visitphilly.com, has a great City Guide to BYOB's.
It's my suggestion that you hang out and take in all the BYOB has to offer. Finish a bottle of wine or two, enjoy dessert and the company you're keeping, maybe take a stroll around the city when you're done. Sometimes just dinner and good friends are all you really need for a night out on the town.
Eat, drink and be merry. Cheers!