Baltimore (BWI) is the most convenient and straight forward. Camp Hidden Valley is ~55min away from the airport and downtown Baltimore is ~50min without traffic.

Washington D.C. (DCA) is ~90min from Camp Hidden Valley

Philadelphia (PHL) is ~ 90min from Camp Hidden Valley

For those on the East Coast, the Amtrak is probably the most convenient as it drops you right in the center of downtown Baltimore or Washington D.C. depending on your accommodation/travel plans.

Take a direct flight into/out of Baltimore (BWI) airport. At time of writing, there is 1 direct British Airways flight per day between Heathrow (LHR) and BWI.

However, this option will likely be the most expensive and least flexible given the limited service. This does not include connection options, which would typically be through JFK, EWR, or DTW airports.

OR Fly directly into New York (JFK, LGW, EWR), Philadelphia (PHL), or Washington DC (DCA) and then immediately take the Amtrak train into Baltimore Penn Station. Sometimes this option can be the most economical as NYC, DCA, and PHL may have much cheaper flights and the Amtrak is reasonable.