Barcelona is beautiful, it reminds me a lot of San Diego in terms of feel/climate. You will have plenty to do / see to keep you busy over five days: Park Guell, Sagrada, and so much more. Museu Marítim is well worth a day. When I was there we took a short train to Ventimgilia Italy just to say we had been in Italy. There are several Roman-era ruins to visit in Ventimiglia as well.
Near far as I can tell, there is a standard "3 countries in 1 day" tour that multiple tour companies offer that pick you up in central Barcelona, take you up to the town of Baga in Catalonia, then up to Ax-Les-Thermes in southern France (seems to be to southern France what Berkeley Springs is to the West Virginia panhandle), then down through Andorra on the way back. If I go, I will definitely be taking that so I can say I've been to Andorra: if Baga and Ax-Les-Thermes end up being interesting that's gravy on top.
My wife and I are big fans of the city building board game Carcassonne, and the French city that the board game is based on is only a 3 hour train ride away from Barcelona. You have to change trains in Narbonne to get there, so why not visit Narbonne too, which seems to have most of its attractions a 10 minute walk from the train station.
Of course this assumes I can simply go from Spain into France and not sure that is true with COVID, or would be true in Dec. If France is closed off, I suppose the alternative for day trips would be Zaragoza and other Spanish cities.
Overall all these places are suitable for "gawk at architecture from outdoors" type tourism that is fairly COVID-safe.