I'm not French dude!
There's some here though, for sure, I hope they chime in.
As for London I can say two things, I liked Camden quite a bit, cool vibe there, more relaxed than the centre which is really really crowded (especially Picadilly Circus and Soho and that stuff, way too much for me) and you should definitely check out the London Castle, it's amazing in daylight and by night, such a beautiful castle.
Oh yeah and there's Denmark Street for really expensive guitar stuff (but cool to check out nonetheless).
In Paris they have a street full of guitar shops as well which is probably even cooler but I can't remember the name.
Ah, googled for a bit and this should be it, three streets that are kind of in a square crossing eachother, not far from Boulevard de Clichy:
Rue Duperre, Rue de Douai, Rue Jean-Baptiste-Pigalle
For food I'd suggest going to Cartier Latin, there's lots of nice restaurants there.
Also in Paris I liked the part where Sacre Coeur and Montmartre are quite a bit.
Oh yeah, and you could go to Pere Lachaise, which is the graveyard where Jim Morrison and Edith Piaf amongst others are buried, it's quite big and has a beautiful park, really a nice stroll.
If you don't go to Paris I'd suggest going to the coast of Bretagne, nature there is amazing and rough, steep cliffs, it's supposed to be quite beautiful.
As for Rome I wasn't too crazy about it but there's some nice things.
Things I'd skip:
Circus Maximus, Colosseum, Roman Forum, Sistine Chapel, Vesuvius: not much to see there / way too crowded / you can't really visit the more interesting areas.
Things to do:
Spanish Steps: really nice park a bit outside town.
3D Church: really cool church that has a flat ceiling but it's painted so from particular spots it looks as if it's not, quite amazing!
Pompeii / Ostia: this was actually one of the more interesting sights around Rome for me, lots to see, kind of like Roman Forum but much better.
Pantheon: one of the most beautiful and impressive buildings in Rome in my opinion.
Well, I didn't think I had much to add to this topic but maybe there's something nice for you in the whole story.

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