You are not alone my friend because many visitors to Seville struggle to decide whether to spend a 3rd day in addition to a typical 2 day plan. I will always suggest a 3 days plan, only for Seville city. Day trips are extra.
A 2 day visit to Seville is a blur
If you stay in Seville for 2 days you will be able to see the main attractions of the city like the Cathedral, Alcazar, Plaza de Toros, Plaza Espana and Maria Luisa Park, and Triana district. You may even have an evening for a flamenco performance and another evening for a tapas route.
But trust me that a 2 day trip will leave you exhausted and not at all satisfied with your visit to Seville. Even after you see everything on your list you will feel that Seville was a blur.
Seville is worth 3 full days
My suggestions is to treat Seville like you would treat Madrid or Barcelona. You need to spend 3 full days in Seville to know the real city behind just the list of main attractions.
Seville's diverse attractions need 3 days
The long list of places to see in Seville has a lot of diversity. So you will certainly not be bored of the same thing. If at all, you will find that just 2 days is not enough to fully see even the main sights.
There are Royal buildings like the Alcazar and Royal Tobacco Factory; each with its unique purpose and architecture. Then there are relatively modern structures like the Metropol Parasol. Of course there is the ancient site of the Spanish Inquisition and the more modern Plaza Espana; again each is completely different. And add to this the Plaza de Toros bullring, the best in Spain according to many visitors.
Just 2 days is not enough for stories of Seville
The Moorish, Christian, and Jewish history of Seville can be seen everywhere is Seville. There are many stories attached to each of the buildings and districts you will visit. And they are so fascinating that you will feel like you need to spend at least a few hours in each place.
For example, The Cathedral Cubiertas Roof tour tells the story of its construction. The Alcazar has a visit where you can see an enactment of the characters that lived in the palace. The Spanish Inquisition museum has its own stories. The convents have workshop tours to see nuns making sweets...the list is long and even if you don't do the full tour of each place, I hope you will appreciate why I say 2 days is just not enough.
In 3 days you see different faces of Seville
Each district of Seville is distinct. There is more to Seville than just the area around the historic centre. Santa Cruz has its own charm as a Jewish quarter with old synagogues and tree lines plazas. The La Macarena is a world of its own where the locals come out every evening into the streets around Alameda to spend the evening.
And you can think of Triana as a city of its own with so much to see and do, with a working class history of ceramic factories and gypsies who lived here in communes. You can walk around all day.
Experiences and tours need a 3rd day
When I think of Seville I think of Flamenco, Tapas, Religion, Bullfights, Inquistion, Ceramics, and so many more things....and there are amazing tours attached to each of these unique aspects.
At the minimum every visitor to Seville will end up going on a tapas bar crawl, and a flamenco performance. That is already 2 evenings gone. With a 3rd day in Seville you can rent a bike and explore the residential areas of Seville, go on a unique experience or tour, or take a short flamenco lesson...do something that is off the beaten track.
Even secondary attractions in Seville are major
There are many Seville attractions which remain hidden to tourists because by the time you are finished with the must see ones, it is already time to leave.
With a 3rd day in Seville you can consider a visit to a couple of sights that interest you personally: for example, the spectacular and world class Fine Arts Museum, the Royal Tobacco Factory, La Isla Magica and Aquarium (with kids), Seville Convents for cookies, Casa Pilatos, Lebrija Palace, Palace of San Telmo...so many more.
So my suggestion is that you must plan for 3 days to spend in Seville. You will not regret it.
Seville is a compact city. You can see most of the attractions in two days, though it will be rushed. The heat and humidity of Seville will make it tiring too. Three days is perfect for Seville.
On the 3rd day in Seville, tell yourself, enough of sightseeing. Spend the day walking around the city as a resident would do on a day off; this is the best way to see the real Seville. The extra day in Seville will also give you a chance to see and eat at places that tourists normally don’t go to.
You can make one exception. If you haven’t yet visited the Museum of Fine Arts, you should visit it in the morning. Seville Museum of Fine Arts houses excellent works of El Greco, Velázquez, etc.
In the middle of the morning (as a way of settling into a relaxed mode) I recommend trying the traditional orange wine in the famous Taberna El Peregil, a pioneer in producing this wine about 50 years ago. This is on Calle Mateos Gago.
For lunchtime, I recommend a small restaurant called "El Tío del Saco", on Calle Trajano. Their tapas are very cheap and avant-garde and their pizzas are spectacular. I recommend the semi-sweet white wine "Canto 5" delicious! As a curiosity, at the entrance have an electric chair (joking) where you can take photos.
If you like ice cream and it is hot, Sevilla has a craft ice cream that is very famous called "La Fiorentina", in Calle Zaragoza. Personally, I recommend the orange blossom ice cream ... exquisite! In this area, there is a very curious shop full of products for fans of comics, movies, etc. It's called "Nostromo". They have a huge Xenomorph head from the movie "Alien". This is the kind of place that my friends and I would like to hang out at, away from the tourist traffic.
In the afternoon you could have a coffee or a drink on the terrace of the shopping center El Corte Inglés del Duque. It is located on the fifth floor and has a spectacular view, with the Giralda in the background.
In the evening, as a change, I would recommend dining at the Japanese Banzai restaurant where they serve Japanese tapas.
After dinner (or even before) you can go to the famous "Micro Theater", which is located on the top floor of the same Banzai restaurant. It is located on Calle José Gestoso, near the Encarnación (the Metropol Parasol).
You will certainly spend a very different day and night in Seville compared to your first 2 days as a visitor.