Paris is lovely for a short break, Nostradamme, Eifel Tower, L'arc de Triumph etc... And the weird metro that has wheels as well as rails (I assume they still do that, it's been about 20 years since I was there)... But I could not imagine spending all that long there.
The Mona Lisa (in the Louvre) is actually just a small yellow painting of an un-exceptional woman and a massive crowd infront of it - you'll get at most a few seconds infront of it - and only then with some assertion.
Paris is a total contrast to the rest of France, it goes from a Western nation to something out of the middle ages. To give an example, in Paris toilets are clean - but you pay to use them. In the rest of France toilets are disgusting holes in the ground, but at least free... The Parisians have the audacity to call them Turkish toilets, like they're not responsible at all for their terrible sanitation!
I could spend a week in France, but I couldn't spend more than 2 or at most 3 days in Paris.
Most French are very pleasant and will help you in English as best as they can (English speaking is relatively common, but extremely heavily accented).
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London is a fantastic place to visit, you must see Hamley's, Covent Garden, Trafalgar Square and have a night out in Soho.
Much like Paris however, it's novelty wears off quickly and the prices are extremely high. You could be entertained every day of the week with something new and different in London but it'll cost a small fortune.
I can highly recommend some time in Holland, if you get the chance go to de Efteling (it beats Disney Paris hands down), de Bazaar is also pretty cool if you head up North - and Amsterdam has my favourite museum of all: Katten Kabinet which is worth a few hours of your time although it's a fair way off the beaten path - the Rembrandt museum is less crowded than Le Louvre in Paris, but if you want to see Anne Franks house you need to arrive EARLY. Very early. The queues are a mile or two long (there is another house of a less famous Jew in Haarlem a few miles out from Amsterdam which never has a queue).
If you head further East Vienna (or Wein) is worth a visit, which has the Prater (amusement park) and some lovely architecture - plus some nearby climbable mountains for some in the wild romantic sex.
Rome is great if you love churches, if you don't then there isn't much else to see. You can see the entirety of Venice in a day and Pizza is a pretty dirty city except around the leaning tower of pizza which is quite nice.
I've not been to Barcelona.
Hope that helps.