I've lived in DE most of my life (I'm 30) and depending on what time of year you're here would dictate which end of Delaware to visit (although northern DE is ~90 min drive from southern DE). I would say the Rehoboth Beach area during Spring/Summer. Definitely make a stop by Dogfish Head (restaurant is in Rehoboth off the main strip, the brewery is a short drive north in Milton) and check out the food (calamari is the best around), beer (really unique/great beers if you haven't tried already, incase you haven't seen Brewmasters on Discovery Channel), and tax free shopping at the outlets on Rt. 1 just before you hit Rehoboth from the north. If you visit during the Fall/Winter, I'd recommend northern DE (where I currently live). There you will find Christiana Mall (again, tax-free shopping...many cross over from PA, MD, or NJ and shop there for that reason) and stop by Iron Hill in Wilmington off the Riverfront. There are twp Iron Hills in DE...the one in Newark is in the heart of University of DE off of Main st., but I find that their beer isn't as good as the Wilmington location. Aside from that, not much else IMHO.
The obvious answer is: to get from Pennslyvania to Maryland by road driven vehicle it is the fastest and shortest route. I live here and I can not give a good reason unless you enjoy nice beaches (East Coast) that have State facilities at user friendly prices which include camping and surf fishing. Wilmington has a fair night life (theater, etc). Here is one, Philadelphia, Baltimore, NY City and Washington DC, and the Poconos are all within a 3-4 hour drive. Try this for a few reasons.
One of the things I love best about this state is its abundance of disc golf courses. I know it's a niche thing, but there are some world class courses here -- Iron Hill, Carousel, Brandywine. I'd say per capita, DE is among the best disc golf spots in the US.