Step under the Big Top. Walk up to any stand and press E to pull the lever on a joke booth, or climb the High Trapeze rig for an obstacle-course to join in the act.
Stand under the center spotlight and press E to leave the tent.
Desktop: WASD + mouse, Space to grab a trapeze bar. Mobile: joystick on the left, drag to look on the right, GRAB! button appears mid-ride.