My best places, and a couple that I would suspect but haven't checked yet, would be
*1, The Beach
*2, Tot Lots
3, Athletic Fields (Have Searched a couple *Volleyball Pits but would think areas around soccer and baseball fields etc., under bleachers would be good also. Problem being all the aluminum supports and cramped spaces. Might cause some difficulty.)
4, City Parks, especially those where there have been festivals or fairs.
5, If in a snowy area, check for where the town dumps snow. In our area, a few towns, clear their down town business districts and parking lots and truck the piles away. Find where they dump all that snow and you might find a lot of lost coin and keys...
6. I have heard that if you can find where storm water is dumped in a local drainage creek or canal... there is often a lot of stuff washed out of sewers. Stuff lost on the streets can get washed in the drains during a rain and then, over time, make their way out of the system.
7, Street and sidewalk projects and construction sites. Anywhere that the surface is graded or bulldozed or where concrete or asphalt surfaces are dug up can expose old items that have been unattainable for many years.
*from my personal experiences.