Extra night/day in Sevilla? What to see?

Absolutely take a day trip to Cordoba. A fast easy train ride. See the Mezquita which blew way most of the otherwise fabulous places we saw on the tour. Have lunch here https://casamazal.es/ for wonderful Sephardic Jewish food.

The tour will also include the cathedral, not to be missed. I spent the night after the tour in Sevilla, giving me that day. Including the free time on the tour, I saw the Triana neighborhood, the Archives of the Indies, the Caridad Hospital, and the Encarnacion Plaza and Roman ruins beneath it. Other choices include the Bellas Artes Museum, the Plaza de Espana, and the Church of the Savior.

Cordoba would be a good day trip, or better overnight. I spent two nights there after Sevilla and liked it a lot, the Mezquita is one of the highlights of Europe for me. If you have only one night after the tour ends, my suggestion would be go straight to Cordoba and spend the rest of the day and night there. It's a fast train ride, and the train will also take you back to Madrid the next day if that's where your homebound flight is.

We stayed in Seville an extra day/night in 2017 after our RS tour. It was a Sunday. We wanted to decompress so we walked across the river to the Triana neighborhood and visited some chapels and the famous ceramic/tile shops. We also happened into a cave museum devoted to the Spanish inquisition. Once back closer to the hotel, somewhere near the bull ring, we found the locals drinking, playing music, and dancing in mid-afternoon, it was fun to see. Most shops were closed, so we walked again to Plaza de Espana and continued down into Parque de Maria Luisa. Sunday in the park with Julio.

Most everything I thought of has already been mentioned. You could take the short trip to see the former Roman city at Itálica. It isn’t that far outside of Sevilla.