What is the best way to spend a day in rainy Barcelona?

I suggest the City History Museum, which is at Plaça del Rei, close to Plaça St. Jaume. There you will see the guts of the city: how the Roman city was organised, where the Romans washed their cloths (btw, using the pee of the citizens, which releases amoniac that whitens cloth), how the Romans made their Fish-based spreading paste "Garum", how they made their wine, etc. and all in the excavated museum under that old quarter Streets. If I have a visitor in Barcelona I just recomend this museum if they only have time for one such visit and want to get the gist of how the city was 2000 years ago. I had the honour of touring John Hennessy, the president of Stanford University in the occasion of his "honoris causa" laureate ceremony at the Technical University of Catalunya (UPC, http://www.upc.edu), and he loved the museum. There is certainly a new space which can be regarded as a museum and is a must now. This is El Born Centre Cultural (http://elborncentrecultural.bcn.cat/en), where the life of Barcelona in the XVIII century just before and after the succession war is explained on the remains of the bombed and knocked down city by the Borbon King army. It is a great place because it is the first time in history when an archaeological site is fully documented because Barcelona has the oldest "registrar" documentation archive (more than 12KM of shelves) dating back to the XII century. El Born is totally linked with the sad history of Catalunya and its claim of independence. It shows how the citizens were just killed and obligued to knock down their homes to build a submitting citadelle. I hope this helps...