I live and have lived near a wedding / function venue on a private estate. You would be amazed at the ammount of valubles / weed/money/wallets etc people drop when going to their car intoxicated in the early hours of the morning. Beach combing in remote areas can also be fun (not really treasure) . Swimming with goggles under fishing spots can yield knives etc. Magnet fishing is cheap but most iron things have no value.
Over in the garage sale and goodwill sections you will find astute members making $ using nothing more than their knowledge.
I used to attend a monthly auction in a rural area for the Lions club. Treasures galore in vintage machinery, butter churns, vintage bikes etc. City folk caught on after a decade or so.
Coin roll hunting - virtually free . Meteorite hunting, magnet on a stick -pretty cheap.
flea markets, classieds etc.
guess its what your definition of 'treasure' is.
Chub