On my first visit to London I liked seeing Westminster Abbey the most. It is amazing to see the effigial tombs of Queen Elizabeth I, Sir Isaac Newton, Chaucer, etc., etc. On that visit I wanted to see all of the sights that you hear about: Buckingham Palace, the changing of the guard, Big Ben, Piccadilly Circus, the Tower Bridge, etc., etc. I like shopping in Mayfair and Knightsbridge -- you've gotta see Harrod's food hall, totally amazing. I'm not fond of the food in England -- the meat tastes odd -- I can't even eat the breakfast sausage. However, there are some great Chinese restaurants, and I like the fish and chips. We always end up going to Hard Rock Cafe a few times -- after so much different food, we're ready for some good old American burgers. Anyhow, it'll be so exciting for you to see things that you've only read about in books.
I second everything BR said about London. It is a great city to go to. . . .once. I can't say I'm anxious to get back.
I'm dying to get to Prague! And I think Paris is an amazing city - but very expensive particularly with the $:Euro. (Our favorite part is siting on the wooden foot bridge over the Seine River at sunset. We'd watch the sun go down over the river with a bottle of wine and a baguette. Magical, really.) I would probably try to hit one old city (Praque or Madrid) and one hip city (Paris or Barcelona) but not all in the same category!
Here are my thoughts on Spain -- It's really my specialty, since I married a spaniard.
Madrid: Old historical city. Really neat, I've been a few times. But honestly, I'd probably skip it if we didn't have good friends that live there -- and it's an easy place to fly into.
Barcelona: I have spent 10 days straight there and it's not enough time to see everything. I LOVE, LOVE, LOVE this city. It's vibrant, alive, beautiful. The Guadi architecture -- particularly his Park Guell -- is amazing and unique. I highly recommend side trips to Sitges or Aiguablava (
http://www.parador.es/english/paradores/ficha.jsp?selparador=1). That website is to a Parador in Aiguablava. Be sure to check out the other Paradors, too. They are amazing castles and monistaries and such that they have turned into small hotels.
Valencia: Just thought I'd give you another city to consider. It's not too touristy and very authentically spanish. It seems to be getting a lot of attention lately -- 2007 and 2011 America's cup. 2008 new Formula 1 race course through the city. The architect Calatrava is from there and he's done amazing things!
Anyway, if you end up going anywhere in Spain, PM me. My husband is a wealth of knowledge and he loves to talk to people about his homeland!