One thing I've not yet written about - the bicycle route along the beach side, from Barcelona northwards all the way to, supposedly, Costa Brava!
I did this ride from Premia de Mar to Barcelona, and back - about 30km from the city center, roughly 1.5hrs by bicycle. It's pretty much perfectly flat the entire way - which makes it more or less an ideal ride for the average person who loves bicycling and seeing the country that way.
Once in Barcelona, things definitely start to become chaotic though - I wouldn't say it's the best city for cyclists, and I hated being on most of the roads, where there are no bicycle lanes and cars seem pretty eager to get around you. There are to big streets however - "Av. Parallel" and "Av. Diagonal" - that feature huge pedestrian and bicycle medians, which you can take for quite a while. If you head on Av. Diagonal going out of Barcelona toward the beach, it's also pretty much a calm and relaxing downhill the whole way - really perfect!
Anyway, I would definitely take some time to do this is you're in Barcelona for more than a few days. Barcelona has city bikes which you can pick up and drop off at locations around town, but you can only use them with a monthly pass, so it's not really worth it. I borrowed a bike from a friend, but there are also traditional bike shops where you can rent one for ~8 Euro for the day.
The only thing to be careful about as you bike along the beach, is that sometimes the bike trail departs from the pedestrian one, and be sure to stick to the bicycle route. First of all, sometimes people actually get upset if you bike on the boardwalk. And then, there's the issue of the boardwalk sometimes falling off into beach/sand ---- so, you'll occasionally have to move away from the beach front a bit, but you'll always move back!
Happy cycling folks,