I would suggest coming into Philly the night before. I think the school asks you to be here around 9, so you don't really want to risk being late if you're traveling that morning. The interviews are mostly done by around 2-3 as well, so you'll still have time to check out the city. I'd also take public transportation; unless you park your car past 41st street, you're going to be subject to parking tickets. But then again, if you're lucky, you can drive around 41-43 streets and the cross streets looking for a spot. (The dental school is right on 40th). If you do take public transportation, you can always leave your bag w/ the concierge in your hotel till you're ready to leave. I stayed in the Club Quarters hotel in center city for about $80. Penn gives you info about it in the interview packet. Regardless of if you drive or not, I would take public transportation to explore the city. Just make sure you have lots of singles cause the buses and subways are $2 unless you have tokens. Hope this helps!
BTW i'm a 2nd year at Penn...and procrastinating studying pathology...if you have any more questions let me know. good luck!