I grew up in Salem, NH, but I live in Upstate NY now...
I remember seeing guys with detectors at Hampton Beach when I was a kid.
I've thought of what places would be good to hunt in the land of my youth... and all I can think about is Canobie Lake Park! An amusement park on a lake that's been in operation since 1902 - thousands of visitors every summer... arcade and carnival games (hello quarters!), concession stands (hello change!) wow!
On the minus side, I doubt they would let anyone with a detector run around in there, but on the plus side, I doubt they ever have! I can't even begin to image the coins in that place.
Just my thoughts!