I've been to New York and Boston for weekend breaks and that's always enjoyable. I've been to Paris and London to sight see but each trip was 8 days. It's far for a weekend from the US. Also I've spent time in Rome, Florence, Venice and Milan. I have enjoyed all of them and do really enjoy big city vacations. San Francisco is another favorite!!
As far as I'm concerned, Nice is a fantastic destination. It's cheap and easy to get to from all over Europe and has beaches, art, good food and a superb old town to explore, plus big city stores.
The climate is good in all seasons - it was the ultimate winter resort for many years. There's a selection of hotels on Travelsavvy-Nice that covers all price brackets - it's also a completely independent guide to the city. And there's Monaco 15 minutes away by train.
SAS, Icelandair and Finnair all offered weekend packages to five capitol cities. Packages included air, hotel and a tourist card allowing rides on trolleys, subways, buses and admission to museums. It was a very satisfying way to visit these five cities briefly, and very inexpensive at the time. I guess I did Vienna, Athens and Istanbul the same way, and Kuala Lumpur, Singapor, Bangkok.....many of these on Gray Line tours which I have found to be good for me.
City breaks.... 
Try Nice for a great weekend break 
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