If you're going to Paris, the Champs-Elysees are a must. Go both Evening & Day if you can have two trips, because it will be well worth it. Don't be discouraged (& move on) with the queues for the Tower, or you'll be forever regretting so (Like me!

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In Paris (More so, than London) goto a cafe (not restaurant) on the sidewalks for lunch or breakfast.
Boat trips for every major city, are again mandatory.
If you enjoyed the Arc then Marble is just as good. Trafalgar & Piccadilly are also must see.
Don't do a 'Fergie' and ask to see London Bridge, but actually Tower Bridge!
Harrods of course, since people have mentioned Covent & Oxford, I'll mention Bond & Regent streets!
St Pauls is nice, Lloyd's building for the unusual.
BUT ABOVE ALL ELSE, MAKE SURE YOU NEVER FORGET THE CAMERA AND USE IT!
