Cheap vacations that are actually possible
You cannot fly anywhere for $400. You can get on a bus, or a train, or into a car you sleep in, and that is a different question with a much better answer.
Three trips, three ways to travel. Every dollar is accounted for, and the assumptions behind each total are printed on the page instead of left out.
By bus
$369
Philadelphia, 4 nights
Four nights in Philadelphia, on a bus ticket and a dorm bed.
By train
$404
Washington, DC, 3 nights
Vacation not found. Found it: three nights in Washington, by train.
By car, sleeping in it
$420
Green Mountain National Forest, Vermont, 3 nights
Four days in a national forest, sleeping in the car, paying nothing to sleep.