Personally, I like to hunt them all... generally speaking I seem to pull a lot of coins from around picnic tables, near snack stands, bleachers and in the kids playground. The obvious places where people gather.
I seem to have my best luck in fields where they play soccer. The fields seem to be relatively free of trash and I find a lot of unusual (but so far, inexpensive) things there (odd jewelery, medals, pins, various arcade tokens, trinkets, dog collar tags and licenses, and even some strange foreign coins). When hunting around town, I just wander the fields un-methodically and have fun (griding is too much like work). I find that by searching randomly I always miss stuff and always know when I go back there's plenty more. It seems like the stuff keeps coming no matter how many times I go back. If you're working an athletic field and you hit upon a coin or treasure, pay a little more attention to the immediate area. It's not that uncommon to find multiple things in a hole. Don't overlook dirt parking areas and pathways that are used.
Don't get me wrong, there are a few fields and playgrounds where I've been skunked and unless It's a real lazy day I like to dig everything. Every piece of trash I take out is something I (or someone else) won't have to deal with on the next visit.
What's nice about many of the kids playgrounds is that they are often covered with wood chips or pine needles and the digging just seems easier. Lots of stuff is right under the top layer hidden in plain sight.
Happy Hunting!