The crater of extinct Volcán Pululahua can be entered on foot via a steep trail from the Mirador de Ventanillas viewpoint just inside the Reserva Geobotánica. It makes for an intriguing half-day trip from Quito. You'll need to sign in at the rangers' office at the entrance before starting the walk. From the viewpoint, a 1.7km trail leads to the crater; allow 90 minutes there and back.
The best time to visit is in the morning; by the afternoon the crater is often obscured by mist.