We had 48 hours in Seville. We toured the Cathedral, old Santa Cruz district, and Alcazar one day. We did the hop on/off bus tour the other day.
While we enjoyed the bus tour well enough, the day touring by ourselves was much better. To get the flavor of Seville, go tour the Cathedral. From there, the Alcazar is next door, and the old Santa Cruz district is right behind. Spend the day wandering the general area. It's wonderful!
-Bob