Public parks can be a great spot to go metal detecting. Really any location where lots of people congregated throughout the years is a great area to do some metal detecting. Before digging anywhere though, you should always make sure to first check the rules regarding metal detecting at that location. Some parks in our area allow metal detecting and others do not.
When planning to go detecting at a public park there is certain equipment or accessories you should bring along with you. I strongly encourage you to wear headphones whenever detecting in a public park. Other people enjoying the park won't have to be subjected to the noise your metal detector is making. It's not only the polite thing to do, but it will also help to greatly cut down on all the outside noise so that you can hear the faint signals and deeper targets. When detecting in a public park you should always use a small sized shovel. Handheld lesche diggers work great. Big shovels not only look intimidating to others, but they make it pretty darn impossible to dig a neat, small sized hole. With a big shovel, you physically just aren't able to dig a small sized hole. The blade is simply too big. In addition to your detector, headphones, and a shovel, you should also bring along a carrying pouch of some sort to carry both the treasure and trash that you find. A pair of gloves will come in handy too and don't forget your pinpointer.
Some areas in a park that I would suggest you check out with your metal detector would be picnic areas, the outskirts around the playground equipment, and any grassy walking paths. I like to look for big old trees as well. If the park has a pond, it would probably be wise to search around the pond. Any grassy areas near swimming pools would be another ideal spot to search. Really any location in the park where people have gathered or continue to gather is a good area to search. I would avoid searching in ball fields all together though. It's a good way to get kicked out of the park and metal detecting banned at that location. The holes you are digging could leave the ground unsteady for those who use the fields and possibly cause potential injuries. I also recommend metal detecting in any high traffic areas of the park during off peak hours.
You'll find that there are a lot of trashy areas in public parks. Searching in trashy areas isn't always easy. I have found that with a little patience and some luck you can be quite successful. I think the key to finding good targets in a trashy area is going slow and keeping your coil very low to the ground. To be most successful, you're going to want to swing your detector at the pace of a snail. When you sweep your coil too fast, you end up missing targets completely, especially the faint and deep signals. Those are the signals you really want to hear too since they usually end up being the best ones. In trashy areas, I always pick up all the surface trash I see. The less trash to contend with the better.
It's probably the opposite from how most people would go about detecting a trashy area, but I like to start out detecting any trashy area with my stock coil, since it's the coil I'm most comfortable using. I select a 10 by 10 ft area to grid. I set the boundaries in my mind and walk in straight lines. I work the area from the direction of north to south, then east to west, and lastly in diagonal paths. When you hit an area from different angles you often can pick up targets you might have not heard otherwise. As I walk along I also try to overlap the ground I previously covered so that I don't miss out on any good targets. If the area is producing a good amount of finds, after I finish gridding the area with my stock coil on, I will switch to using my small coil and search the area again using the same gridding process. A small search coil can provide you with better target separation.
Please remember to always check the park rules before detecting, dig responsibly, always fill in your holes, and take the trash you find along with you or dispose of it in one of the parks garbage cans. Happy hunting!