Cordoba is a must stop if you ever visit Sevilla.
Although the old town of Cordoba resembles lots of similarity to Sevilla, the Mezquita is something special of its own. You can find a lot of Roman influence in this Mosque.
Here is what I recommend as for your itinerary, taking an early train from Sevilla to Cordoba, strolling through the old town and exploring the Mezquita, Walking across the Roman Bridge and capturing a nice shot of the bridge with the town in the background. Make sure to enjoy your lunch in the old town and then catch the train back to Sevilla.
As for the afternoon, once you get back to Sevilla, go check out the Plaza de Espana, once a playground of the World Fair of 1929. The Plaza is magnificent which we ended up paying a second visit the next morning.
I am wearing:
Top: Nordstrom (see similar here)
Skirt pants: Made by my grandpa for my mom (see similar here)
Sandals: Vince Camuto (see here)
Bag: Chloe (see here)
Earrings: Amazon (see similar here)