What can I say about London's Hyde Park. It's stunning!
For me, this one of the top places to visit on your London trip. It's perfect in the summer, but suitable for all times of the year. Enjoy a nice walk, or take a bike ride around the park. Take a little walk down via the lake and grab a coffee (or whatever you want) from the cafe by the lake and take in the amazing views and sights that Hyde Park has to offer. In the summer time on a sunny day, it's perfect, other times you will find events, such as concerts and a perfect example of this time of year (November / December) is Winter Wonderland. An amazing event.
Since I have been living in London, I've lost track of the times I have come for a nice morning walk at least twice a week. I can't recommend this enough.