Fine!
Tell her you've recently sold your soul to a pimp named Pepperjack and must work the streets of Philly to pay him off.
Tell her you have volunteered to participate in a reenactment of the signing of the Declaration of Independence.
Tell her you're going because you bloody feel like it and that she should stop being so nosy because you're not her little girl anymore, you're all growed up.
Tell her it has been your lifelong dream to be in the Guiness Book of World records so you're going to Philly to help set the world record for world's biggest orgy.
Tell her you're running away from home because she won't let you wear make-up.
Tell her you have been diagnosed with butt cancer and the leading butt cancer specialists are in Philadelphia so you have to go there to seek special treatment.
Tell her you have an online boyfriend who lives in Philly and you're going out there to elope.
Tell her the Phillies have invited you to sing the national anthem at their home game.
Tell her you've recently gotten into hunting humans, and the only place it is legal is in Philadelphia, on the specific weekend that NEST also happens to rest on.
Tell her you're going to a music festy.
Tell her that her butt stinks and you need to get away from it for a little while, and Philly seems like a good destination.
Tell her a psychic told you were going to die that weekend, and the only way to survive was to escape to Philadelphia.