Areas under the trees, especially if they are old. Even before it becomes a park maybe folks where under those trees for shade, etc. Re matts under playground woodchips etc. I'm certain Ive had good targets under those , and the weed fabrics there etc. I do not disturb the stuff and leave the target be. It is there to protect the children after all. Reshuffling some wood chips on top is one thing, but messing with the base material to me is a no, no, and would probably not be considered a trite thing by park officials. I judge each baseball field differently. There are some ancient ones out in the small or big towns that are probably mowed 4 times a year and then there are those that get weekly grooming , watering, feedings with nightly games, pa systems, lighting etc. The better ones may have dedicated fencing around them, and are even posted. I think you should leave those better ones alone. In this upstate city the finest place for metal detecting was a centrally located large historic and stately park-the city jewel and central social gathering place for probably 130 years. There was no problem with going there. But two years ago it was put off limits to diggers. As one of the grounds persons told me, the league, fenced in ball fields, where torn up by someone with a metal detector digging, leaving holes that kids where tripping on etc. Hence we all got put out because of that one or two who had to go in there and disrespect the place. Along sidewalks are good. In old parks look for evidence of where the main paths and sidewalks used to be. Sometimes like theyll put up a new bath house in a new location and knock down the old ones, but you can still figure where they used to be from old pictures and historical societies. Ive been having good luck also with the hills for sledding. And also Ive had a lot of luck with a certain park where they would like for 100 years now annually flood certain field for public skating etc. Good hunts, CO