The Cavalcade of Corvettes will be in Philly this year,the sponsoring club has the show every year and moves it from time to time.Easy access via I95 and/or the PA Turnpike and just a few minutes from Parx Casino which may also have horse racing that day for those so inclined.The past 2 shows I've attended had over 300 cars each. I suggest if you go try to get their early as it's almost always hot,sunny and shadeless.
http://www.ccdv.com/forums/statements.php?action=Cavalcade_of_Corvettes