Depends on how mineralized the ground is. I run my sensitivity as high as I can without a lot of false signals. If I get a lot of falses, I turn it down until they go away. Increasing sensitivity in highly mineralized ground can actually lose depth, I have heard. Too much in the ground for the signal to bounce off of.
As for discrimination, I run mine as low as I can get away with. I discriminate iron only. I'm not anywhere near a civil war zone, or any other battlefield, and I've already dug more than my fair share of farm implements, so I could care less about Iron. Unless, of course, I'm at a Ghost Town.
A really trashy park you may want to increase your discrimination to notch out pulltabs and lower. Keep in mind you may be giving up a gold ring for this, but you'll dig less trash. Depending on your detector, it may also lose some nickels too.
Too get the gold rings, dig 'em all. Maybe a thousand pulltabs or so to each gold ring, maybe a hundred, who knows? But if you notch tabs out, you will probably miss gold rings.