Be wary of gladiators outside the colosseum. They don't take photos for free and can be quite agressive about their payment. Also don't be fooled into taking an unlicensed guided tour of the Colosseum either....
London is one of the BEST places to go shopping! If you came to shop, you MUST treat yourself to just a little something from the TopShop on Oxford Street.
For a great view of the city (especially at night) go to the top of the Eiffel Tower at Paris. It also offers a great vantage point for viewing the water show at the Bellagio.