Hello! I have 2 days to fill. Checking out of Barcelona on Monday, May 12 and checking into Madrid on Wednesday, May 14.
Thank you in advance
We spent 4 nights in Valencia between stays in Barcelona and Madrid. Valencia is a great city. It is much more laid back and less crowded than the other two. Unfortunately, I think a lot of travellers give Valencia a miss when they try to fit in Seville with the other Big Two in a tight schedule. For example, with only 2 nights, you could consider a more direct and shorter route by staying in Zaragoza instead. Good luck.
I'd choose Zaragoza, with the caveat that you could be unlucky and hit an early heatwave there. Zaragoza is a very interesting city, but it's oppressively hot in the summer. Normally, mid-May isn't bad, but the heat arrived early in 2022, with mid-May having highs in the 90s (F). Zaragoza is right on the rail line between Barcelona and Madrid, so you won't spend any extra time on trains to get there.
Valencia is also worthwhile, but it involves a good bit of extra transportation time; the detour down the coast will add about 2 hours' time.
I don't know what your plans are for the rest of the trip, but Toledo is worth a couple of nights, if not three, so another possibility would be to travel from Barcelona to Toledo, changing trains in Madrid.