The most active volcano in Europe and by far the biggest dominating attraction in Eastern Sicily, Mount Etna is a must visit. If you head to Etna Sud you have the option to walk around some of the older craters on foot or take a cable car ride to 2,900m, from there it is a 2-4 hour round trip to the summit craters, although there is no need to go that far to get a good feel for the majesty of the mountain. The other point of departure is Etna Nord (Piano Provenzana) from where you may walk or take 4X4 buses up to the observatory at 2,400m. You may walk to the summit craters from there. I would highly advise to not go venturing to the main craters of Etna without a qualified guide.
The only active and severely destructive volcano in Europe, Mount Vesuvius stands tall at 1281 metres. Having erupted several times and destroying the cities of Pompeii and Herculaneum on one occasion, Vesuvius could well pour its wrath yet again on the thousands of people living around its crater. Walking 900 metres up to the apex could be long and strenuous and the winds could aggravate your troubles, so do go prepared with jackets, bottles of water and walking shoes. Additionally, go only if you are really interested in looking at the crater as there is nothing exceptionally special about the destination, the prices are a little too high and the guides meet you for a minute and give you lesser information than brochures available at tourist offices. You can however expect great views of the bay of Naples from the top. If that wasn't enough of a redeeming factor, I still have fragments of Mount Vesuvius inside my shoes. Getting there: Circumvesuviana trains run from the Naples Central Station to Pompei Scavi-Villa Dei Misteri and the ride is around 40 minutes. You might not get a seat in the train so beware of pickpockets. You have to shell out around 6 Euros for a two way journey and it is a good idea to start a little early in the afternoon. Tickets for Vesuvius are available at the Pompei Scavi train station itself and come for around 20 Euros, inclusive of everything. Buses run every 40 minutes right outside the station and take you to the entrance of the Vesuvius National park where you are transferred into a 4WD style bus which drops you 900 metres below the crater and you are given 90 minutes to complete the trek. Expect a super bumpy but thrilling ride. The last ride is at 4 pm for most of the year, so do keep track of your time.