Some suggestions:
- In Paris, don't forget to go to Montmartre area, where the Sacre-Coeur is. One of the most romantic place on earth, where Renoir often got his ideas for his paintings. Also if you like paintings, it is worthed to visit Musee' du Louvre, and perhaps the Musee Picasso, housing the most outstanding Picasso works (duh!)
- In Rome, visit Santa Maria in Cosmedin, by the Isola Tiberina. It is most often ignored by tourists, but then you would regret it, because it is one of the tradition you have to do besides throwing coins to the Trevi - put your hands to the Bocca' della Verita' (the Mouth of Truth). If you have time, attending the Greek Orthodox mass here is unbelieveable. You are in time machine back to fourth century mass!
To understand layer of Rome, visit San Clemente church south of Colosseo. You keep going down, underneath the modern street level - as the 16th century church was built on top of 4th century early church, which had been built on top of an ancient Roman temple. You can see arches literally emerged from the floor.
I agree with ajuhdg - drink as much water fountain in Rome as possible. Nothing is like you experience before, drinking from water fountain in Rome.
If you're in Rome during the Pantecost - visit Pantheon at noon, where you can see red rose petals falling like rain through the occulus of the church. The most amazing thing you've ever imagined.
- In Capagnia region, Positano and Amalfi always come to mind. Riding a bus along Amalfi Coast to Positano or Amalfi, makes you respect the early Italian engineer and the bus driver. Never seen anything bluer than the Mediterranean. Also visit Capri, see the Blue Grotto where all you can see is a natural cave with natural blue light comes from underneath the water.
- In Prague, visit the Old City Hall every hour to see the apostle's parade of the clock. It was cheesey, but it was fun at the same time. Tee-hee
Also visit the Vinohrady area, it is NOT a tourist area, but I just don't understand why none of the tourist books mentioned Vinohrady in more details - this is where you can see least tourists and experience Prague to the fullest with locals. See the Church of Sacred Heart in Vinohrady 2 by Jose Plecnic.
- In Amsterdam, if you have time, you need to get out of the city. Yeah, Amsterdam is cool and yadda yadda, but there are more cool things outside as well. Go to Sans Schans (sp?) to see the authentic Dutch Windmill - most of the windmill throughout the Holland were in here, and some of them are still active, and you can enter the mills to see how they made the oil or color essence. Or you may go to the Northern Dam, which WAY cool when you realize that the water levels are different on each sides of the 30 km+ bridge. Or visit Delta Project. Makes you wonder ...
Have fun in Europe. Watch out for pickpocketers, especially where people don't have any personal bubbles.