Hi Lee, went to Circuit de Catalunya about 20 times in my F1 days (long story), but always stayed in the nearby town of Granollers, which is really rather nice and chilled. Went into Barcelona once, to work on Barca v Man U, worst night ever (even longer story), but we drove in and don't remember much of the in and out. There's a train station in Granollers and pretty sure it is a Barcelona commuter town so bound to be a train from there into the city or from nearby Montmelo. Not sure how you'd get to either, i walked to Granollers from track once and it was a bit of a trek, but there might be shuttle buses or taxis. Have a good time.
