St James's Park
St James's Park between Buckingham Palace and Horse Guards Parade is one of the royal parks in London.
At the centre of the park is a pond, with a crossing bridge which is a popular photography location. The park has two cafes, where you can relax after walking around the city. The park itself has lots of squirrels, which are accustomed to people and will take food from you if you offer it. The pond and the surrounding area has cranes, swans and pelicans.
You can also hire deck chairs between March and October if the weather allows it. The chairs are £1.80 for the first hour and £9 for the whole day.
The Green Park and Charing Cross underground stations are both 5-10 minutes away.