From Pas de Bellecombe, it's possible to hike up to Balcon du Dolomieu, a viewpoint on the northeastern side of the Dolomieu Crater rim, from where you can gaze down upon the bottom of the caldera, some 350m below. Balcon du Dolomieu is a five-hour, 14km out-and-back walk. It's graded moderate and has an altitude gain of about 500m.
It can get very busy, but the eerie landscape more than makes up for the crowds of people. Note that since the 2007 eruption it's no longer possible to do a circuit around the Dolomieu Crater and the Bory Crater; ignore all maps showing this circuit, as they're not up to date.