Prefer the wet sand (saltwater and freshwater) Gonbechan.
Re. places to go. I suggest, like in fishing, search at your feet first e.g. your own garden.
Beaches, well, they change from tide to tide and wave / wind conditions.
One location I’ve never tried, but would love to, is at Fuji Rock in Niigata. Can imagine someone in a few hundred years, when the concerts and skiers have long gone, discovering that place with some fancy machine designed to pick out the desired metal. How about searching with a drone that does the digging too?!
It is a fun hobby for sure. If Señor and his bro are still going out regularly in a few years time, then they have passed the point of no return, it is terminal.
Many will give up due to all the rubbish though. Metal detectors = weapons of trash elimination. Nice white sand at some places, but underneath is another world. They may also find after digging e.g. ten beer cans from one hole, or bags of obento rubbish, that humans are pretty dirty and lazy animals.
Keep it as a hobby and avoid the gold fever mentality is my suggestion. In the case of Señor, the real ‘gold’ is possibly to get out there with his brother. Priceless!