I don't know how much help I'm going to be..I don't actually live IN Philly so I don't know too much, but some places are..
69th street - Shopping..they have a whole lot of stuff there. That's pretty much all shopping
South Street - is much bigger than 69th street, with a whole lot of things. Stores, restaurants, clubs and all that kinda stuff..(always fun to just walk around) real nice
Penns Landing.. I'm sure something will be going on around Memorial Day.
Something I just found on their website..
Still the Official Kick Off to Summer in Philadelphia, Captain Morgan's Jam on the River at Penn's Landing celebrates 20 years in 2005. This 20th annual event will again take place Memorial Day Weekend, May 28-29, on the Great Plaza at Penn's Landing.
http://www.jamontheriver.com/
and if you're into clubbing..
http://www.geocities.com/tecknophreak/phillyclubs.html
you can go there and see some clubs
And you should go to Eviplist.com and check out the events going on in Philly around the dates you're going. (it won't list anything around that time probably until the week of) They list all the parties and who's hosting them. CWebb and AI hosted a couple recently. Something is always going on though
I don't really know much of anything else..good luck and have fun in Philly
oh and don't forget about the cheesesteaks..