You cannot come to Barcelona and not see Gaudi's cathedral masterpiece. Truly awe-inspiring. When I was there was only half completed but should be much nearer to completion by now.
Couldn't be easier. Sagrada Familia has it's own metro station (strangely enough called Sagrada Familia) on Line 2. Just catch the metro heading towards the eastern terminus of Line 2 at Pep Ventura. The cathedral is a five minute walk from the metro station
Updates
Send your feedback or queries to been.there@guardian.co.uk
Search Been there
Don't refer to the SF as a 'cathedral'; the cathedral is in the centre of town and taxi drivers will take you there. The SF is a church. More quibbles later...