By road
All states in this region maintain convenient service plazas along the major highways, which are probably your best bet for hitchhiking as the freeway itself is closed to pedestrians.
By car
When in Rome, do as the Romans do; drive in the USA. Driving is the only practical way to tour the Mid-Atlantic, if you intend to visit anything beyond the major cities. That said, the Mid-Atlantic is the least pleasant place to drive in the entire country. Traffic, both inter-city and intra-city, can be horrendous—the D.C. area regularly tops the lists of the nation's most congested cities, and Metro New York is never far behind. But above all, Mid-Atlantic drivers are rude. Aggressive and often puzzling driving is commonplace along the interstates and in more populous areas.
I-95 is the coastal super highway, at times encompassing a whopping eighteen lanes, and connects D.C. to New York through Baltimore, Philadelphia optional, and pretty much everything in between in New Jersey and Delaware. It is the fastest way to drive between these cities, but it is unpleasant and ridden with expensive tolls especially in Delaware! and very bad traffic on weekends and around local rush hours. Avoiding I-95 requires a bit of creativity and slightly longer trips through Pennsylvania west of Philadelphia, but such trips can be rewarding in terms of scenery, no tolls, little traffic, and just easier traveling generally.
Finally, a direct warning is in order: Driving into D.C. and New York is really not advisable. Doable, certainly; fun, not likely. Washington D.C.'s street layout was designed to confuse invading armies. New York is extremely congested, and its drivers are the most aggressive in all of North America though Bostonians may dispute that claim. Philadelphia is also filled with extremely reckless and fast-drivers, but at least the city is laid out in a grid. If you are driving into D.C., get good directions and don't do it anywhere around rush hour. If driving into New York, save yourself by parking in a less congested borough like Queens or Brooklyn, where you might actually find free on-street parking, and take a subway line from there into Manhattan. Driving in Mid-Atlantic cities is really not recommended however because it is not necessary; Mid-Atlantic cities have the best mass transit systems in North America. It is almost always better to take a train in and use mass transit than to drive in and pay astronomical parking expenses.
By bus
Greyhound (http://www.greyhound.com/) is unfortunately the main intercity bus operator, unfortunately because it is an inefficient and expensive way of getting around. Fortunately it is seeing low-cost competition from a peculiar set of Chinatown bus routes. These no frills point A to point B services arose organically from the needs of Chinese-Americans to visit their relatives in the various Chinatowns across the region. The service caught on when the general public realized they could travel round trip from D.C. to New York for $30. There is no central internet site for these services, so you are best off doing a general internet search. Be aware that the Chinatown buses do occasionally terminate and originate in unsavory urban neighborhoods.
A new wave in intercity coach transportation is large corporations trying to emulate Chinatown buses but with better services. Megabus (http://us.megabus.com) was the first to embrace the new business model, offering heavily discounted fares from its New York hub to Boston, Toronto, Buffalo, Philadelphia, Baltimore, Atlantic City and Washington. Bolt Bus (https://www.boltbus.com/d...) is a division of Greyhound but is significantly cheaper and more comfortable than their regular services, they provide newer coaches wit more legroom, electric sockets at every seat and free wireless internet. Bolt Bus provides journeys within the northeast corridor, almost identical to Megabus. Ne-On (http://www.neonbus.com/) is also a division of Greyhound and operates between New York City and Toronto, via Buffalo, their on-board services match those of Bolt Bus.
By plane
Although there are lots of budget flight options between cities on the Mid-Atlantic, due to the hassles and delays associated with increased security precautions, especially in the New York City area, short flights are usually best avoided. All major cities are connected by regional and commuter rail options with stations in the central business districts of these destinations. Moreover, most inter-city planes travel between suburban airports fairly far out from the city center. When the distances are long enough, though, flights can indeed be cheap and convenient, from, say, Washington, D.C. to Albany, or New York to rather far-flung Pittsburgh.
By train
While train service is very limited in the rest of the United States, the Northeast has by far the best rail service in the country. The high-speed Acela express runs frequently along the Washington-Philadelphia-New York-Boston corridor. Amtrak also serves upstate NY, and western PA. Unfortunately, Amtrak is a very expensive option, often more so even than flying or renting a car!
In addition to frequent Amtrak service, there are a number of much cheaper regional commuter rail lines including: MTA Metro North (http://www.mta.info/mnr/) and Long Island Railroad (http://www.mta.info/lirr/) around New York City, NJ Transit (http://www.njtransit.com/) in New Jersey, Shore Line (http://www.shorelineeast.com/) for Connecticut, SEPTA for the Philadelphia area also connecting to Wilmington and Newark in Delaware, and MARC for Maryland especially the Baltimore-Washington, D.C. corridor.
It is possible to travel much of the coast using only regional rail, and although it will probably be slower than the Amtrak or Acela Express, it will also cost a fraction of the price, and may take you through towns you might not otherwise see. Making the trip from New York City to D.C. is relatively straightforward, with only one rail gap between Philadelphia and Baltimore: take New Jersey Transit from Penn Station to Trenton Amtrak Station, connect to SEPTA R7 Regional Rail to Philly's 30th Street Station, get from Philly to Baltimore by other means Greyhound bus or car rental being the most simple way, then connect to the Marc Train at Baltimore's Penn Station to D.C.'s Union Station. Note, though, that Amtrak leverages its political power to monopolize the rail lines to an extent on weekends, shutting down the popular Marc Train.