1.give at least one day to Philly, two days will be perfect - one day for sightseeing in downtown area and one day for shopping: They don't have any sales tax on clothing stuff and there are a lot BIG malls in the surrounding areas. I am kind of regretted that I did not shop a lot when I was in Philly.

2. Two to three days in NY. last time when I Stayed in Philly, I took the train to NY, very convinent. It will be difficult and probably expensive to have a parking spot in NY city.

3. two days in DC and on the way to DC you can visit the longwood garden too. it is a 3-hour's drive from Phily to DC.

4. One day to Baltimore if you are interested in this city, close 2-hour's drive from philly.

DC worth one day( if u want the national zoo, half additional day);

baltimore inner harber is only worth half day;

philly only worth one day.

Longwood garden is worth visiting if it is on the route,

if u need more than 2 hours half way, reconsider it.

Princeton in NJ is a very nice town if you have time. Doubt the fountains at Longwood garden do not open in winter, then there are fewer things to see there, only greenhouse.

NYC is definitely worth 2-3 days. 1 day for DC. 1 day for Philly. 0.5 day for baltimore i think is enough