Sevilla was the most beautiful city that I visited in Spain. There are a ton of things to do, and the Alcazar was the top highlight for me. You can book a "free" tour of Sevilla, but be sure to bring cash for tips!
As a solo traveler, I felt pretty stressed though because Sevilla was already quite touristy in May and seemed better suited for groups. For example, a lot of the tapas were served in sit-down restaurants, whereas Madrid and San Sebastián had more tapas bars. The restaurants were super crowded in May, so I would definitely recommend making reservations if you don't want to wait.
If you're short on time, you can probably skip Triana. It's a nice neighborhood, but there aren't as many things to do.
0:00 - Intro
0:37 - Archivos de India
0:56 - Museo de Bellas Artes de Sevilla
1:12 - Plaza de España
1:43 - Hotel Alfonso XIII
1:46 - Bar Dos de Mayo (one of my favorite tapas experiences in Spain)
2:05 - Parque de María Luisa
2:21 - La Carbonería (free flamenco shows run every half hour if you buy a drink, recommend getting there early because it gets very crowded)
2:45 - 80s Bar
3:14 - Coffee
3:19 - Puente de Triana
3:25 - Capilla de los Marineros
3:35 - Real Parroquia de Señora Santa Ana
3:39 - Mercado de Triana
3:44 - Thrifting
3:50 - Catedral de Sevilla
5:11 - Lunch tapas
5:24 - Real Alcázar de Sevilla
6:11 - Dinner tapas
6:22 - Beautiful spring flowers