Reminds me of when I first picked up on park detecting. It was quite frustrating for the first few months until I got a handle on what good deep coin tones sound like, and at what depth to expect to find them at in any given area. A local park that I had initially detected over period of a couple of weeks, many years ago, provided nothing more than a few shallow decimal coins, and a lot of junk at first.
Then one day I hit my first pre-decimal - a florin, and the coins seemed to flow after that. What I learnt was how a coin tone should sound at depth, say compared to a shallower target, and what size targets I should be looking for when pinpointing (most were reasonably faint, and pinpointed as a small target compared to shallower canslaw and other obnoxiously loud non-ferrous items). Turned out that I had been walking straight over all those faint deeper coins, and only picking out the more obvious shallow targets, most of which was crap.
If you use target ID's, coin numbers should remain reasonably stable, whereas with irregular shaped items (ie. Canslaw) the numbers can jump around by several digits, despite sounding good. The deeper the coin, the less accurate your ID will be, hence you should rely more on your tones for accuracy on deeper coins.
Some parks in the city are littered with so much modern junk, you are better off walking away from them to save on frustration, especially when initially getting your head around on how your detector operates.
The beach is probably the best and least frustrating place to learn on initially, and still manage to walk away with some coins for your effort (or jewllery if you are lucky). Tot lots are heavily hit these days, so you might struggle to find a paying one. Hills tot lots have always been more fruitful than city ones.
Also do keep in mind if a cemetery has been turned into a park, it is more than likely that the whole area has been top dressed with soil, adding extra depth to what would already be reasonably deep pre-decimal coins in the first place.
The Adelaide Hills has been good to me regarding pre-decimals, probably more so that in the city, especially river bank frontage. Some areas are found with some research, whilst others are found with blind luck.