In reply to Greenbanks:
All of it, really. The stuff in the crater is near the visitor centre and the cable car etc, and Arico is a pretty village with a cafe and the gorge is easy access and a good place to walk. Also the place by the main road (can't remember the name, begins with G I think!) which is accessed between the two tunnels north of Las Americas will, when fully developed, be brilliant.
I've been twice, it's great fun and the weather is always lovely. Not too hot at this time of year.