If you're only going to be in Paris for 3 days and this is your first time you might consider avoiding the Louvre or Versailles.  Those take up a lot of time if you want to truly want quality time there.  If you do like museums I heard the D'Orsay one is good.  Go check out Sacre Coeur (Sacred Heart).  You can either choose to pay to take the funicular to the top or brave the steps.  The view is breathtaking and, to me, well worth the effort of going up the steps.  Then swing over to Montmartre, the kitchy artist community right next to the church.  And cheesy as it is you should visit the Eiffel Tower and Champs Elysees with the Arc de Triomphe.  A nighttime dinner cruise down the Seine River is nice.  Make sure to visit Notre Dame, another beautiful cathedral.  Grab some ice cream at Berthillion.  I can't remember if it's on the same island as Notre Dame or one of the other ones.

With London the only things I can remember are Hyde Park, Buckingham Palace, and London bridge.  I've been to Paris twice so I have a better memory of that!

http://community.thenest.com/cs/ks/forums/thread/69828377.aspx

There is a lot to do in London, but the must see IMO is St. Paul's Cathedral. 
It is truly breath taking.  I got the Rick Steves London book from the library and followed a lot from there.

I have a couple posts in my blog for Paris....I went with my students and we only had a couple of days (we did other parts of France as well), so we hit most of the highlights.  What are your interests?  What do you enjoy doing/seeing? (museums, monuments, etc).....what I consider a must do might be different for others

One of my favorite things is to climb the towers of Notre Dame because the view is amazing and you can get up close to the gargoyles, etc.

I'd say skip Montmantre and Sacre Couer - the church is beautiful, but the area that surrounds it is dodgy and full of pushy annoying people who try and sell you stuff.  I'd definitely hit up the Louvre instead.  I would highly, highly recommend a dinner cruise on the Seine - we used Bateaux Parisiens (sp?) and it was definitely the highlight of our trip. Pricey but so worth it.  Also check out Saint Sulpice if you have time.  That was my favorite church in Paris, even better than Notre Dame I think. 

 In London, check out the changing of the guard and there is a museum in Trafalgar Square (I can't remember the name) where you can get high tea.  FInd some good Indian restaurants because the London stereotype is true - the food is awful!!! 

 

Just got back from London. We did a Big Bus tour, which made stops in several places and you get off/on at your leisure. It also includes free walking tours and a boat cruise on the Thames. We did the walking tour for the Changing of the Guard and toured the Tower of London (do the Beefeater tour) and Westminster Abbey. All were def. worth seeing.

http://www.bigbustours.com/eng/london/custompage.aspx?id=london_sightseeing_tour&gclid=

 

 

Awwww...Paris....what I would do: Trip to Versailles (this will probably take about 6 hours, round trip) - without a doubt, our excursion to Versailles was my favorite part of my trip to Paris. Pick up the Rick Steve's Paris book - we would not have been able to fully experience Versailles without it. I would skip the museums, honestly, but you can get in out and out of the Louvre pretty easily to see the Mona Lisa if you have to (Rick Steves can help you with that as well). A dinner cruise down the Seine is an amazing way to see the lights of Paris...and we loved, loved, loved the Moulin Rouge. We attended a 11pm showing so we were half drunk from dinner but...it was a really, really good time. If you are a designer wh0re like myself, make sure you hit up the flagship Chanel, Dior, and Louis Vuitton stores. So fun.

MrsD2006:

Just got back from London. We did a Big Bus tour, which made stops in several places and you get off/on at your leisure. It also includes free walking tours and a boat cruise on the Thames. We did the walking tour for the Changing of the Guard and toured the Tower of London (do the Beefeater tour) and Westminster Abbey. All were def. worth seeing.

http://www.bigbustours.com/eng/london/custompage.aspx?id=london_sightseeing_tour&gclid=

 

 

DH spent about 4 days in Paris once on a trip back from visiting family in Turkey, so for us it was the relaxing part of a long trip. We rented bikes from the Velib stations located all over the city, only 7eruo for a week! We had to use DH's cousin's credit because it wouldn't take ours (just FYI). We rode around the city every day,and exercised off all the crepes we had for breakfast on Rue Cler, an adorable street super close to the Eiffel Tour (and our hotel).

We did ridiculously stereotypical things like buying bread, cheese and snacks from shops and eating dinner on the lawn under the Eiffel Tour all but one night. We did not go inside a single museum, but the outsides were pretty :) We found it to be a nice city to relax after lots of family time.

In Paris, you have to spend a whole day at the Lourve Museum. It's beautiful and of course there is the Mona Lisa. You have to spend a whole day there though. I only spent 2 hrs there and skimmed about half.

 

In London, all I did was shop  :)

operaghost:

I'd say skip Montmantre and Sacre Couer - the church is beautiful, but the area that surrounds it is dodgy and full of pushy annoying people who try and sell you stuff.  I'd definitely hit up the Louvre instead.  I would highly, highly recommend a dinner cruise on the Seine - we used Bateaux Parisiens (sp?) and it was definitely the highlight of our trip. Pricey but so worth it.  Also check out Saint Sulpice if you have time.  That was my favorite church in Paris, even better than Notre Dame I think. 

 In London, check out the changing of the guard and there is a museum in Trafalgar Square (I can't remember the name) where you can get high tea.  FInd some good Indian restaurants because the London stereotype is true - the food is awful!!! 

 

operaghost:

I'd say skip Montmantre and Sacre Couer - the church is beautiful, but the area that surrounds it is dodgy and full of pushy annoying people who try and sell you stuff.  I'd definitely hit up the Louvre instead.  I would highly, highly recommend a dinner cruise on the Seine - we used Bateaux Parisiens (sp?) and it was definitely the highlight of our trip. Pricey but so worth it.  Also check out Saint Sulpice if you have time.  That was my favorite church in Paris, even better than Notre Dame I think. 

 In London, check out the changing of the guard and there is a museum in Trafalgar Square (I can't remember the name) where you can get high tea.  FInd some good Indian restaurants because the London stereotype is true - the food is awful!!! 

 

operaghost:

I'd say skip Montmantre and Sacre Couer - the church is beautiful, but the area that surrounds it is dodgy and full of pushy annoying people who try and sell you stuff.  I'd definitely hit up the Louvre instead.  I would highly, highly recommend a dinner cruise on the Seine - we used Bateaux Parisiens (sp?) and it was definitely the highlight of our trip. Pricey but so worth it.  Also check out Saint Sulpice if you have time.  That was my favorite church in Paris, even better than Notre Dame I think. 

 In London, check out the changing of the guard and there is a museum in Trafalgar Square (I can't remember the name) where you can get high tea.  FInd some good Indian restaurants because the London stereotype is true - the food is awful!!! 

 

I could not disagree with this more. The montmarte is one of my favorite districts in the city and is arguably the most "real" and un-touristy area within the actual downtown of Paris.

In Paris, what I really like to do is wander through the streets and markets...it's a cool city.  I love the Latin Quatre for it's bookshops and the small food places you can enjoy.  Musee D'Orsay is a fabulous museum, and much more doable than the Louvre (which is amazing, but not necessarily worth it for such a short stay).

London - there is so much to do (and as I live here, I know it better).  One new thing that has come to London is the cable car ride over the Thames.  It's very cool and much better than the London Eye.  If you are in London Thur - Sat, I would hit Borough Market for lunch - meat eaters should try out the chorizo sandwhich guys, non-meat eaters should go for the  Raclette cheese stand (I think they are still there).  Definitely do a hop-on-off bus tour - you can use the ticket for 24 hours.  And I love, love, love the London Walks (www.walks.com) - have yet to go on a bad one, and I go every year for at least one (which means about 9-10 walks).  St Pauls is a great church to visit, and while insane, you should check out Oxford St & Regents St.  If you can, see some music at St Martins in the Field, a small church in Trafalgar Sq - it's so quaint and fun.  For food, I'd check out www.toptable.co.uk for reviews & to make reservations.  Have fun!