We will be in Paris for three nights, and London for three nights. Any must see recommendations for a first time visitor? One adult and one 16 yr old traveling together.

I would start by looking at a guide (at this site for example) then making a preliminary list.

for example. if you love art, then it would not be a 'waste' to recommend a Louvre visit for your limited time. But if you are not into museums...

for a short time, I might consider taking a half day when you arrive to do a coach tour overview of the city, then going to your specific interests. For Paris, for example, I might not take time to go to Sacre Coeur in your short timeframe, but a bus tour that went there for a 15 minute stop, OK

That being said....Paris:

Eiffel Tower, Seine river, Louvre and area (Louvre, Tuileries, Palais Royal,Place de la Concorde, Champs Elysee, Arc de Triomphe) , Notre Dame and area (St Chapelle, ND, Ile St Louis), Places des Vosges and Marais area (shopping too)

London: Tower of London, St Paul's, Covent garden, Trafalgar Sq (museums?), Big Ben, Westminster Abbey, St James Park, Buckingham Palace,...

The options are really endless - take a look through some guidebooks together and see what grabs your interest. I like using the Michelin Green Guides. It really depends on what your interests are - history, art, gardens, etc...

I agree that a half day bus tour of London and a Seine boat cruise in Paris would be a good way to get a preliminary look at major sights.

But you must do some research on your own before you go to get an idea of what things appeal to you. And make sure your 16 year old travel companion does some research, too. We find that lots of planning adds a lot to our enjoyment. Get a good guidebook - we like the Michelin Green guides and the DK Eyewitness guides. Check them out at your local library.

Once you have gotten some ideas of what you want to see and do, come back here with a draft itinerary and questions. You'll get lots of ideas.

London- Tower of London, Westminster Abbey & British Museum
Paris- Musee D'Orsay, Notre Dame and go get ice cream at Bertillon on Isle St Louis (sp), the 16 yr old might like the catacombs; one that I haven't made it to yet, but I hear wonderful things about is St Chappelle

I do recommend a guide book...Fodors and Rick Steves print very useful ones

You can only begin to scratch the surface of the two cities, so fi you can give us some guidance..

It all depends on what is important to you:

historic sites
art (and what kind)
music & entertainment
shopping
food

If you can give us a sense of your priorities, we can make better suggestions.

SS

The Rodin Museum in Paris is wonderful (but the Picasso Museum is closed, I believe - watch out!) The Marais for shopping, the 11th for a more "everyday" Paris feel. . . the Jardin des Plantes in spring is great as well.

Obviously there's nothing of importance or interest in London or Paris, otherwise you'd have known at least a few places you wish to see.

I've heard that a good travel website like, say, Fodors.com, could have some information that would help you. This is especially true because at this point, only you know your interests.