I tentatively have a trip to Madrid in February. I have spent considerable time there and want to explore other places. I will have 3-4 days to spend elsewhere.
While I have spent approximately 36 hours in Barcelona and saw some of the sites, but obviously couldn't cover everything. I did love it there.
I have already spent time in Sevilla, Costa del Sol, and Toledo. I've spent a day in Cordoba, Gibralter, and Grenada. Never been to Valencia.

My favorite activity in Spain is just hanging out. Spent a ton of time sipping wine in Retiro park in Madrid, along the river in Sevilla, and Ciutadella park in Barcelona. I don't really need major sightseeing to enjoy a city.

Leaning toward revisiting Barcelona, but am always interested in seeing a new place.
Open to other ideas as well. Would like to keep it to a train trip, but willing to jump on a flight out of Madrid.

(ETA: This all assumes Covid has calmed down by February, of course)