The museum of prehistoric Thera is also worth a visit if you make it there! It is fairly small but contains some incredibly well-preserved artifacts. The frescoes are stunning.

I found Fira (nearby "large" town on Santorini, very close to the sites) miserably crowded, but seeing Thera was very cool. This is all about 1500 years older than Pompeii, so a very different experience IMO.