hehe, having spent my first 28 years in the UK, we spent a lot of time travelling around as kids... we had a saying "seen one castle, see 'em all!!" :^)
The West Country (Devon & Cornwall) has some of the nicest countryside & beaches... my mum still lives in Plymouth, and one of my bro's lives in Torquay. I lived in and around Brighton, and if I ever went back to the UK, I'd head back there (or the West Country), it's only an hour or so from London on the train, so a pretty easy day trip. I also grew up in the Oxford area which is pretty cool for looking around some old buildings... and of course you have to try 'Punting' while you're there!
Amsterdam is a pretty cool city.. you've already had the main advice there! There are lots of boat tours around the canals too.
Paris is ok... I'm not a big France fan though.
DO NOT LEAVE SPAIN WITHOUT TRYING TURRON!! I cannot stress this enough!!! Being part-spanish, we used to get sent this every christmas by relo's. I still get some sent over whenever I can. There are several different varieties available, the soft nougat (blanco I think) is the best! Or the chocolate almondy one! Oh man, I'm salivating just writing this! If only I could buy it here!
Have a lot of fun!