I would try a park or your front yard. Unfortunatley, It was common practice to bury garbage from the construction of a house. My house was built in '68, and I found paint cans, bits of flashing and ductwork buried in the clay before landscaping was done. I dug it all up so I could have clean "testing grounds" close by.
I find on most detectors, foil or balls of wire will give readings that jump around.
Also double targets also do the same. Drop some coins,rings,pulltabs and bottlecaps to learn their readings. Coins usually quite constant. I sweep 90 degrees to the sweeps that jump around to see any change.
I would start in standard mode to learn your detector. This may irk the loyal AT Pro guys, but I think you should know, there are a few posts regarding AT Pro owners who found out they have a defective early build that made false noise when their coil hit a blade of grass or at the end of their sweeping motion. Other times it would only detect huge targets and miss coins. (I constantly dropped coins to test that the detector was working. Sometimes it wasn't)
Lastly the detector would start dinging wildly like a slot machine jackpot.
I experienced all 3 of these symptoms within my first 10 days with an AT Pro. I can't help but be jaded that the manufacturer didn't properly test the first batches and then neglect to recall the bad machines from sellers shelves when a known issue has been around for over a year. I love my other Garret products.