My suggestion is pretty simple, have a daily item/treasure hidden somewhere around the entire plaza (not just the beach) to find using the metal detector.
The hidden item could be guaranteed something worth over a specific amount of units 5k-10k+ etc to make the hunt worthwhile and rewarding.
This could also be included in the “Daily Tasks/Awards” update coming in the future and overall gives an additonal use for the metal detector apart from the “treasure on the beach” event.