Public places and places where you find people regularly are always hot spots as things are always falling out of pockets waiting hidden in grass or sand just out of sight for you to come along and discover. Finders keepers that the way it works out of someones pocket and into yours. So some of the best places would be public parks and recreational facilities. These are particularly good just after any public holiday or special event as more people will have visited meaning more chance of things being lost. Busy beaches are a wonderful place to find lost articles some new some old, even car parks at scenic view points can hold hidden gems waiting to be discovered. Think about the metal detector type you have as well as some are better kept away from salt water. Dont use a detector in water unless it is designed to be used their otherwise you will permanently damage it.
Public places where people hang out today looked good but places where people used to go or lived and worked can also be rewarding especially when hunting for older artefacts. Ghost towns, forts, old industrial and dockyard sites can have lots of hidden goodies. These places are very often overlooked just because no one goes there anymore. When exploring these sites make sure that you are not going onto private property or you may find yourself fleeing from a guard dog or two.
Doing some historical research can turn up some great places that havent been explored for years or even decades. Historical groups can be contacted in your area to get useful information from and they are always keen to talk about historical places so you wont have to try hard. Archived copies of old maps and newspapers are also a great source of information on what used to be in and around an area. So visit the local library and see what you can find in their archives that could put you on the route to some valuable finds. I hope this helps you to discover some more places to try out your detecting skills.