POD on the Eurostar between London and Paris. It's brilliant. Book tickets online in advance, print before you leave, scan at the station. You'll clear the French border in London, and the British border in Paris. Takes less than 2 1/2 hours, is comfortable and remarkably quiet. You can buy Metro tickets in the food car, unsure about the Tube. The only downside is that it's not really suited for large quantities of luggage and/or big cases as the luggage racks are very small and it's hard to heave them up onto the train. We stashed our huge cases in the food/bar car.
Multi-day museum passes are your friends in Paris. Don't try to do the Louvre in one visit - it's just too much.
Don't underestimate the impact of visiting old European churches. Everybody goes to Notre-Dame, but how many have been to the basilica of St-Denis? The royal tombs are stunning, almost as spectacular as the stained glass.
The cheateau of Versailles is included in the museum pass, and is a short commuter train ride away. Go early - you can easily spend a day in the gardens. I've been twice, and have yet to make it to the Trianons. Expect huge groups of Chinese tourists.
Watch out for the tacky souvenir hawkers near the major landmarks - they're a lot more aggressive than they were 10 years ago, and they WILL get into your personal space. Always watch your purses and pockets.
If you're a photographer, Paris is God's way of telling you "Learn how to bracket." Take extra media with you. Take the fastest glass you own, the best zoom you own, the best low-light glass you own, a Gorillapod, and at least a circular polarizing filter. If you own neutral-density filters, bring them, too. In summer, the days are very long and perfect for blue-hour shots.
It's been almost a year since I was in London - many segments of the Tube were closed due to renovations, requiring bus rides (with detours due to road works) between stops. The traffic congestion was unbelievable.
It costs 16 pounds to visit Westminster Abbey, but it is not to be missed. I was able to attend a small prayer service at the altar of St. Edward the Confessor (where William and Kate retreated after their vows). Downside? No photography allowed.
Wherever I travel, I come up with 3 lists:
A: Must-see or must-do, bucket-list worthy stuff
B: Sights or activities that I would really like to see and/or do
C: Nice to see or do, but wouldn't break my heart if it didn't happen
I try to learn where all of these things are, and their proximity to other items on the list - then decide on a starting point and take it from there, wherever my feet, the Tube or the Metro takes me. There will always be something to do. Break the day up into 3 or 4 chunks and figure out what you would like to do, at your own pace.
Watch out for labor strikes - they'll usually be annouced with several day's warning. Some can be very inconvenient, such as public transportation or, in my 2nd trip, bank couriers (it's always fun when the ATMs haven't been restocked)
Essential: Good shoes. Foam tape for blistered toes and/or heels. Aleve.
Try to book a hotel that has a tub you can soak in.
Wonderful, wonderful French wine with dinner or lunch may be cheaper than Coca-Cola.
You'll walk so much that you burn off all those wonderful, delicious calories in Paris.
Feel free to PM me with any questions.