1 day in London

Is this is on one ticket (e.g. your ticket is US to Paris via London)? If so, absolutely do not skip the LHR-CDG leg of your flight. If you do that, the airline will cancel all subsequent legs (e.g. your return trip to the States). You could have to buy a new ticket or pay a large penalty. There is plenty to do in Paris; even though some museums are closed on Monday, lots of other things will be opened. I'd go straight to Paris, even if you happen to have separate tickets.

Is this your only chance to see London on this trip? Assuming no airline complications, IMHO you should opt for the time in London.....Use your Sunday to check out the front of Buckingham Palace, walk the Mall, see the exteriors of Parliament and Westminster Abbey, walk past the London Eye and down the south bank towards the Globe, cross the Thames on the Millenium Bridge, check out St. Paul's, have a fancy dinner or visit the Sherlock Holmes pub near the theatre district. This day would be packed but memorable. Even if you feel a bit jet-lagged, the walk should revive you. And if this is your first trip across the pond, you will probably be on an adrenaline high............If you stay in the Bloomsbury neighborhood, you would have time on Monday morning to duck into the British Museum (free) to check out the Elgin marbles, the Rosetta stone, and a couple of other galleries.....The Bloomsbury neighborhood, full of hotels, is charming yet still fairly close to St. Pancras station for your train next day. Be sure the hotel you pick will store your bags when you arrive on Sunday, even though your room will probably not be available in the morning. ..If you want to do this, buy your Eurostar tickets ASAP because they get much more expensive as you get closer to train time.......

Just to agree that Laura and Cynthia are both right. If you can change your flight ticket (or haven't bought it yet), you can spend a day in London. I wouldn't do this, because the first day of arrival is often a jet-lagged haze, and I wouldn't want to be running around London this way. But DON'T just skip the London to Paris portion of your air ticket, or the airline will immediately cancel ALL of your remaining segments, and you'll have to buy a last minute ticket home (costing thousands of dollars). If you can't change your ticket, you'll just have to "suffer" with an extra day in Paris. Look at your tour online, and find the "Pre Tour" tab. This includes a list of places that your tour will not be going, and you can see one of these.

I too have to advise against this plan - I think you're trying to do too much. Your tour will be a whirlwind of places and sights and I'm not sure trying to squeeze one more city in is worth this hassle. There are few cities better suited for a free day than Paris. It is a beautiful city and a no schedule day of walking the Champs-Élysées, shopping Rue Cler, or wandering a market day followed by Luxembourg Gardens picnic with no pressure sounds like one of the best days ever. If you're afraid you won't have anything to do there are some places tours will never take you - the Sewer Tour is interesting and my favorite site no one ever talks about is the the Crypt du Notre Dame which is a tour of the ruins that Notre Dame is built on top of. Of course there are a bunch of more main stream things like many museums, cafes, world class department stores and much more. Just my US$.02, either way have a great trip!
=Tod