I have not been "yet" to Valencia, but I did spend a week in Sevilla. What I loved about Sevilla is:
The cathedral and Giralda Tower.
The Alcazar and it's impressive garden.
The Bella Artes museum, is a must see art museum.
Flamenco Shows – I went twice to Casa de la Memoria de Andadus, on Ximenez de Enciso 28. Tickets are around 10 euros and get there early as seating is first come.
Tapas Bars - Sevilla was the first Spanish city I visited and was introduced to Tapas bars here. For me I like the tapas bars of Sevilla better than Madrid. I hope no one in Madrid read this.
The Barrio de Santa Cruz district – fun district to walk around and stop by the tapas bars found here.
Plaza de España – even thought it is run down a little still has impressive tiles and views.
Walking the streets of Sevilla.
Most of all the people I found there were friendly and fun.
After spending five days here, when I departed for Granada I felt could have spent another week here.
(I would like to here from those about Valencia, should I visit it to on my next trip to Spain?)
Bill