Depends on your local council as some councils have banned detecting. There are other topics on this and I'll try throw up some links.
Basically they are public spaces and OK to detect with a bit of common sense. I NEVER down sports playing surfaces. That's a sure way to get the attention of those who would ban us. Stick around the edges and under tress where people sit.
Try to learn some less destructive techniques like coin popping and the slit method.
This guy is a bit long winded but shows some good recovery methods.