200km London bike race, around that roundabout that goes by Big Ben and the Parliament Building. Timer chips will be required and will count the number of laps. Participation is limited by the number of bicycles that can fit in the roundabout. Strategies include pushing your way to the middle for the shortest distance ride, and riding on the outside to avoid the crowds.

20km San Francisco hill climb, up the twisty part of Lombard Street, down the stairs.

10km Venetian canal swim. Nose clip required.

(This idea inspired by a tiny turning circle near my house. I was on my way to map out a 3 mile running route, and wondered if I should save time and just run around the turning circle some number of times. Adding a mass of people for a race seemed appropriately ridiculous for this place.)