There are many opportunities. In a city most of them are connected to our consumer behaviour, since here most of the time we are dependend on getting things like food, clothes and energy from 'the outside'. But there are also a lot of possibilities to become more independent and live in a more ecologic way which we can create ourselves.
It is very important to realize that no matter where we live, we are embedded in the processes of nature and have an effect on them and that thus, the action of every one of us will have an influence.
City and nature are not seperate.
We humans and our cities are living parts of nature and completely dependend on her.
But often we first have to reestablish a connection to natural processes/pay attention to them, to be able to realize that.

List of simple possibilities to live more sustainably / ecologically conscious