Funivia dell'Etna runs a cable car from Rifugio Sapienza (1920m) up the mountain to 2500m. Once out of the cable car you can attempt the long walk (3½ to four hours return) up the winding track to the authorised crater area (2920m). Alternatively, you can opt for a 4WD minibus to take you up to (and back down from) the crater area; the minibus option includes a 40-minute guided tour of the crater area.
If you plan on walking up to crater, make sure you leave yourself enough time to get up and down before the last cable car. The ticket office accepts credit cards and cash, and admission for children aged four years and under is free.