Santiponce might be the smallest place I have been to in Spain, with just 8 km², but it is really nice to walk through.
Of course, you don´t go all the way here to just see a small place, no you will go to Itálica. Itálica is an old Roman city, just 10 km from Sevilla. In this small town, scenes of Game of Thrones were shot. Italica has applied for Unesco World Heritage status and it is free to visit for European inhabitants. You can see all the Roman ruins here. If you visit Sevilla, Italica is a nice place to visit for a few hours.
Besides that I have eaten the best Paella in Spain at La Caseta de Antonio, so have your lunch there and if you are in Italica, also visit the monastery (Monasterio de San Isodoro del Campo) of Santiponce. It is one of the only things you can visit there, but definitely worth it and also free to enter.
There are multiple buses going from Sevilla (San Bernardo station) to Italica, which only takes 30 minutes.