We're huge fans of the RSPB's initiative Make a home for wildlife. Over on their website they have tons of ideas and activities to do either in your garden, on your balcony or in your neighbourhood. You can choose what type of environment you'd like to work in, what type of nature you're looking to support and how much time you've got and their site will suggest a number of activities.
An easy one we started with is to grow flowers that bees love. It’s suitable for nearly anywhere that gets some sunlight: a balcony, a garden, an unloved spot in your neighbourhood. The main thing to keep in mind is the season and the type of flowers. For summer Allium (especially Allium sphaerocephalon), borage, catmint (Nepeta) Cirsium rivulare, foxglove (Digitalis) and most herbs will throng with all manner of different wild bees.
It’s a great activity to do on your own or with the family. Do share your pictures once you've done them with the hashtag #homesfornature.