Primarily, you will want to search with you metal detector at places where a lot of people have been. And, you will not just want to confine your research to current locations where there are a lot of people. Find out where people congregated in the past.
Here are some suggestions:
Find out where the state fair or carnival has been held for the past 50 years.
Search where the old train station used to be.
If your city is putting in new sidewalks, hunt the torn up area before they lay new concrete.
Search the grassy area between the street and the sidewalk.
With permission, search old farms, churches, houses, and other private property.
Hunt in shallow rivers and creeks.
Detect near the raft or the rope swing at the old swimming hole.
Camp sites - especially if they are used by local boy and girl scouts.
Baseball, football, soccer, and track fields.
Under the bleachers.
Around big, shady trees.
Old battlefield sites.
Behind baseball backstops and dugouts.
Around picnic tables.
Boat docks and ramps.
Outdoor concert areas.
Remember to check local metal detecting rules and regulations and get permission when hunting on private property. Laws vary from state to state and city to city. You may or may not be allowed to metal detect in public parks or you might have to get a permit. Check before you dig.
Re-fill any plugs or holes you dig and take away any trash you find. Always leave an area in better shape than when you arrived. You want the area to look like you have never been there.