Relativity Park sets c to 5 km/h and turns you loose in a 3D park on foot, so effects that are normally unmeasurable — time dilation, length contraction, Doppler-shifted color, the Terrell rotation of a passing shuttle — show up just by walking fast toward a lamppost. A live readout tracks your velocity, Lorentz factor, and how much faster the world’s clocks are ticking than your own watch. Free, keyboard-and-mouse, runs straight in the browser.