Clissold Park

Saved from builders in the 1880s by a local campaign and opened to the public in 1889. Two of its ponds are named after the men who ran that campaign. There is a Georgian house in the middle, a small herd of deer behind a fence, and the New River running through on its way into the city. Compact, well used, and rarely without a dog in it.

Hapoo counts the walk you were doing anyway

Read off your phone's motion sensor after the fact, so there is no start button and nothing running in your pocket. Characters turn up at real park gates across London, this one included.