I don't know anything about that Renfe pass, but I recall earlier complaints about difficulties using a Spanish rail pass (not necessarily that one). I hope someone else can help.
It would be useful to know your full itinerary before suggesting a way to use the extra day. In isolation, I'd suggest Girona. It's an historic city with a partially-walled medieval district and a number of interesting sights (two historic churches, an art museum, a Jewish-history museum). You can spend hours just walking around the historic center. There are definitely other options from Barcelona, but to me Girona is #1. It appears, however, that the nighttime temperature in Girona could be quite a lot chillier than the average in Barcelona. Do you have adequate time planned for Seville, Cordoba and Granada? I'd focus my time there on a February trip, because I love warm weather in the winter (though Granada's at altitude and likely won't be as warm as the other two cities.)
If you'll be approaching Barcelona by train from Madrid, you could instead stop for one night mid-way in Zaragoza. Madrid and Zaragoza are likely to be considerably chillier than Barcelona. Seville and Granada are nearly guaranteed to be the warmest of all.
There's tons to see and do in Barcelona; there's nothing wrong with having 5 nights there.
Don't forget there are quite a few sights in Barcelona for which you'll want to have tickets in hand (or on your phone) before walking up; ticket lines can be long and individual time slots, at least, can sell out in advance. The problem sights are La Sagrada Familia, Parc Guell, Casa Mila/La Pedrera, Casa Batllo, Picasso Museum and Palau de la Musica Catalana. The issue at the latter is that the English-language tours can sell out. Yes, having so many sights essentially requiring pre-purchased, timed tickets that tie you down does limit your sightseeing flexibility. That's one reason it can be stressful not to have much time in Barcelona.
