One of my favorite things to do in London is to simply walk. I have just walked through Hammersmith High street and into/around Ravensourt Park. Unlike Hyde Park and St James park, you will rarely find tourists here and it's always possible to find a good spot to sit. Right now I'm perched on a bench with a group of young professionals playing with a frisbee behind me and a handful of toddlers running around the play area in front of me.
This is a dog friendly park with areas that are closed off for dogs. There are a number of couples walking their toddlers during their lunch break. A beautiful atmosphere in the summer.
A great park to get away from the hustle and bustle of the City centre. There are tennis courts, basketball courts and a tea house within the park.