Visiting the crater of Guagua Pichincha, an active volcano nor far from Quito, Ecuador, which had a major eruption as lately as 1999, can be one of those "once in a lifetime" experiences. I've had the privilege of hiking down to the caldera twice - once in 2009, and the second time a few weeks ago, in Aug 2014. Our latest attempt was blessed with perfect weather. Not a single cloud, until our return to the rim, where a potent ice pellets storm awaited us. Beautiful distant views of the Cotopaxi from the crater rim, and great views of the calderas throughout the course.
Very few agencies and guides in Ecuador will agree to taking you down to the crater of Guagua, for a number of reasons. First - the hike is treacherous. It doesn't require any technical skills, but some stretches do present some mild exposure that should discourage the novices. The part where you transition between the "old caldera" and the "new caldera" can be uncomfortable for some (the "cresta de gallo").
The second reason is that this is a tricky hike. If you are lucky, you get to the Guagua refuge on an AWD (4x4) vehicle - you'll be starting your hike at about 4,500 meters. You'll then hike up to the rim - a short 15 minutes climb which takes you to an altitude of about 4,600. From there, you're hiking down to the crater for a long time. The bottom of the caldera sits at about 4,000 meters and even lower. Keep well in mind that the ***only*** way out is to hike back up to the crater rim. There's no alternative. If you aren't in good shape, you'll be stuck down there for the foreseeable future.
Also - there's only one known route out of the crater (see the GPX file).
Our guide - Jaime Gallardo from Zona Verde, an old friend - gladly took us down there and back. We also took the time to explore some of the lesser visited areas of the caldera (assuming anyone else except Jaime ever goes down there - he had to sort of convince the guardian at the refuge that it was ok for us to go...).