A scale model of the solar system embedded into the city pavements, designed so that a normal walking pace equals the speed of light at the same scale. Start at the sun by the Egan Center on Fifth Avenue; you reach Earth eight minutes' walk later, and Pluto five hours and twenty minutes further on in Kincaid Park. Each planet is marked with a plaque showing its size to scale and a few facts.
It is a clever way to make the absurd distances of the solar system actually graspable. The full walk follows the Coastal Trail for much of its length, so you can do it as a half-day outing combining astronomy with the city's best path.