What should I do with an extra day and night between sevilla and cordoba

It would be helpful to know the rest of your itinerary and the time of year.

Going to Ronda is going to take a lot of travel time. There may be a direct bus from Seville, it's likely to take around 3 hours. It's about the same by train, AVE to Malaga or Cordoba (!) and then another train from there to Ronda. It's almost 2 hours by train from there to Cordoba. It's less than an hour by train from Seville to Cordoba.

I'd add it to Cordoba! One night stays can be a challenge and Cordoba is a is a great small city.

Because its so nearby and on the way, you might consider Carmona and its Roman site of Italica. We only had lunch and a town walk in the old town of Carmona, but it was quite attractive. It might be a nice and quiet but good-food evening. There's a Pousada to stay in. Jerez and Arcos are two other possiblities, but in the wrong direction. It depends on how much you like the highway. Except for those who like to lament the hordes of day-trippers, nobody regrets going to Ronda. It is nice to spend the night in the air-conditioned Pousada-parking is extra.

Wondering how many total days you are traveling and where does this extra night fall?
Thoughts: if you are on a long journey then recommend staying the night in cordoba and taking the foot off the gas pedal of travel so you can relax. Nothing wrong with becoming more of a local.
However, if you are a "must be on the go" personality then recommend a visit to Ronda as it provides a different perspective of Spain. Strongly suggest you go to the bottom of the gorge to see a different view of ronda.

I don't think it's too optimistic, but it is a bit of running around. I think it depends on your personality as a traveler: do you enjoy cramming in as many sights or do you prefer a more relaxed pace? (By the way, I love Ronda!)

A second night in Cordoba sounds worthwhile, and you'd feel less pressure to depart Sevilla first thing in morning.

Cordoba, for Pete's sake.