Richmond Park

The largest of the Royal Parks, and the only one with several hundred free-roaming deer in it, which is the one thing to know before a lead comes off. They are wild animals and they are big. During the autumn rut and the early-summer birthing season the park asks everyone to keep well clear of them. Outside those weeks it is the finest long walk inside the M25.

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.