Amsterdam- the flying pig seems to be the place to go if you want to party and stay with younger people. That being said, the red light district is a dive and a bit sketchy. Check out a live sex show and make sure you go to the 4:20 Cafe or Hillstreet Blues for some special times. If you head out of the red light district you will see the city clean up considerably and it gets much nicer....some cool parks, etc.
London- Westminster Abbey and St Pauls are a must see.....absolutely incredible. You have to go to the top of St Pauls too. Other places are the Imperial War museum (world class museum with amazing exhibits), the Tower of London, Buckingham Palace, Shakespeares Globe Theater and the London Museum.
Spots to see- Piccadilly, SOHO, and all cool spots along the Thames.
All in all, what a cool friggen city.
Barcelona- The cleanest hostel in the city is Pere Tarres. It is right in a modern commercial district and the building is virtually brand new with a ton of rooms. I met some super cool people there that I eventually ended up travelling up the coast to Nice with in a car. It is a transit ride away from La Rambla (an absolute must go every night).
You should also see-
Gaudi Park (unreal place- world famous park with incredible architecture)
Segrada De Familla (see it on the bus tour)
Coastal Fortress (cannot remember what it is called)
Christopher Columbus Museum
Other points- Take the bus tours as they are a great way to see the city with professional tour guides and you can also benefit from discount tickets and maps of all the cool spots. Take tons of pics and be the biggest tourist you can be- spare no expenses- enjoy it for all its worth because you probably wont go back anytime soon. And if you can, book the hostels from this website as they are the cream of the crop for accomodationsl.
http://www.famoushostels.com/
Have a wicked trip David!
"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around the banks will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered."
-Thomas Jefferson 1802