Here's some thoughts:

Plan open jaw flight (fly into London, out of Paris or vice versa). Take train in between.

First day in a city plan a do it yourself walking tour of major landmarks (example London along Thames to see London Eye, Big Ben, Westminster then through the horseguards parade to Buckingham Palace or Paris along Seine to take in Notre Dame, Arc de Triomphe and Eiffel Tower). You will get your bearings and fresh air and exercise is the best antidote to jet lag.

Other days mix things up, we like a 3 point plan such as a museum, park and market. Not three museums in one day.

Check out local publications for things to do and eat and see. We like Time Out in London and in Paris (www.timeout.com). A good small guide book is the "Encounter" or "Pocket" series by Lonely Planet for various cities. The chapters are by neighborhood and limited to highlights of sightseeing, eating, drinking, shopping so great to carry with you for day.

www.londontown.com and http://journeyplanner.tfl.gov.uk/user/XSLT_TRIP_REQUEST2?language=en are great help.

you can request Ceremony of The Keys at Tower of London - free but you have to request well in advance.

How fun! I've been to both in November. Best time to hit all the pubs!

For London sight-seeing, I recommend Tower of London, Churchill War Museum is a must, and Windsor Castle is a very easy day trip on the train from London. If you like the theater, get tickets for a show (or check out the cheap tickets when you get there)--so much to see there. Hyde Park is also amazing. Get a map of it, so you know what you want to see and go on a day that is better weather. Won't be fun at all if it's windy or raining.

Go to Harrods... so beautiful and fun to look around on every floor if you are a shopper.

In regards to pubs, there is an abundance. I would say it depends on where you are staying. I usually ask the hotel for this info (before I leave)--ask them for a choice of good pubs and restaurants in the area. There are the older pubs and the new gastro pubs. You should hit both.

Be sure to do tea. I love the Wolseley. The tea is great, but you can also go for a fancy brunch there. I think lots of celebs hang out there, so it's good people watching, a beautiful place and the food is spot on. You'll need to make reservations well in advance--though the web site or your hotel can do this.

As for Paris... check out the different neighborhoods. Find a market. I enjoy going to Cafe Marche for a cafe and croissant mid-morning and watch the Rue Cler market being set up for the day. Luxembourg gardens is also great to walk around in--to see French families and people watching. Again, choose a better-weather day. I would choose a couple museums you are really interested in and go to those. The cruise on the Seine is nice. For, me Paris is about the food. Ask your hotel about good restaurants in the area.

I've also eaten at Cafe Marche (Rue Cler) at night. It's good, inexpensive and lively. Perfect for 20-somethings. If you want to kick it old school, try Allard. Very old, has a traditional menu and loyal following (need reservations). Lauduree on the Champs Elysee is a beautiful place for pastries and coffee.

If you are looking for something to do at night, maybe go hear some jazz. Again, your concierge should be able to help with that. It's always nice to go somewhere close to where your hotel is.

I love to do something a little different but local when I travel. Last time my husband and I went to Paris, he went for a straight-razor shave at the oldest and most famous barber in Paris. It was fun to find it on the back streets of Marais and see only French people.

Enjoy the planning--so much fun!