Retiro Park, Madrid
The place is huge with many gardens. Coming and walking under the shadows of the trees is wonderful, especially in those hot summer days.
Madrid has a few decently large parks, the one a few blocks from the Prado Museum is really nice, there is a large lake, lots of performers, and plenty of seats to relax.
Enjoy yourself with the people, walk around the lake or hire a boat, see animation groups for children...