re: Barcelona--Madrid--Porto

Been in Spain (Seville) for over 3 years, and have spent a good amount of time in Madrid, Barcelona and Porto.
In Barcelona, you need to visit the Banksy Museum - great way to spend half a day. Also, the Catalonia National Museum is fantastic and def worth a 4-5 hour trip. For food, highly recommend a middle eastern tapas place, Zirvab. Great Indian food at Swagatam. For great blues/jazz, check out Jamboree which is right off Rambla.
In Porto, there's a great tiny wine bar (seats maybe 8 people), Morro d' amores, located at Rua da Bainharia no. 99. Family that owns it has the 2nd oldest winery in Portugal, and they have wines at this bar that are unavailable anywhere else. Its up a steep hill in the heart of Porto, close to the river, so bring comfortable shoes, but its worth finding - also has some small plates for snacking. Spent about 6 hours in there one night last Nov. and it was an amazing experience.
Not the biggest fan of Madrid. The art museums are extremey nice, but everything else tends to be slighty touristy and lacking any true Spanish feel to it. Highly recommend that you spend at least one day in Toledo, which is reachable by train from Madrid (one hour) and has tons of great sites.
If you find your way further south toward Seville, let me know and I can provide tons of recs.