The general rule is that you can photograph mostly anything you like as long as it’s in a public place. You do not need to have permission to photograph in public, this include photographing building and parks as well as people that are in public areas.
If you’re on public property you can even photograph private property, for example standing on the street and photographing someone’s garden. If you are on private property you can photograph until someone ask you to stop (a sign prohibiting photography counts as well) and you must obey such a request.
There are exceptions to this rule, for example military installations or other subjects that can be classed as national security. However infrastructures such as bridges are not included in such a list. Another exception is, even in a public space, places like dressing rooms, restrooms or people entering their code at the ATM machine — photographing at these places counts as invading a person’s privacy.
So to summarize, here are some examples of things you can photograph, if you’re in a public space you can photograph adults, children, law enforcement officers, accidents, criminal activities, celebrities, airports and train stations.