It might be out of the way for most people but Norwood Park next to Crystal Palace is a gem. Big park with amazing views of Central (think Primrose Hill but southern aspect) and ridiculously quiet, even on a lovely day.

The Avenue Gardens area at The Regent’s Park has a lovely shaded stretch with lots of benches. I don’t have the best photos, but it’s beautiful in person.

Crystal Palace Park (though shout out to the Norwood Park comment!). There’s just so many different areas to the park, good for those wanting some shade (like me!)