This is meant to be the most popular tourist attraction in London.

It goes on land and in water and run all year

I was walking through Hammersmith recently and they have a bike bike rack there with some yellow bikes on it called 'Duck Bikes' You can borrow them apparently and ride them down to some wildlife reserve down by the river. It seems to work on some kind of honour system where you leave them back to the bike rack. There was a basket on the front with a map to the reserve etc

Website: http://www.londonducktours.co.uk/

Hop on Hop off Busses

When you are a bit jetlagged and don't want to push yourself too hard, consider doing a hop on/hop off bus tour.  Alternatively, ride at the top of a red double decker bus.  £1.00 on your Oyster card is much cheaper

Website: http://www.bigbus.co.uk/

Cost: £22.00 

London Walks

London Walks are hugely popular and run year round, rain or shine.  We really can't rate them highly enough:  

The "London Explorer" day is a great orientation to the city.

"Old Westminster" which gave us a discount on the very interesting Cabinet War Rooms which we visited after the walk

"The Beatles Magical Mystery Tour" where we end up at the Abbey Road crosswalk. Spouse and I loved this.

"The Along the Thames Pub Walk" which was more interesting to us for the history than the pubs themselves because we likely wouldn't have wandered into the south bank of the Thames if we hadn't gone on the walk. If you take this walk, I recommend going on a week night when the pubs might not be so jammed as they were when we went, which was a Friday night. Big mistake there.

We also did the "Cotswolds and Oxford" day trip and the "Stonehenge and Salisbury" day trip and enjoyed them very much. When my kid spent a semester in London, she went on a slew of walks and day trips with them and enjoyed all of them.

I did the "Jack the Ripper tour". It was fun, we had an interesting tour guide. I'd recommend doing this one at a non-peak time as well.

Also did the "Old Hampstead Village" tour. Liked that one a lot, too. Got to see the Mary Poppins house and where a number of celebrities live (Boy George). Enjoyed the history about the heath and just getting to see a different part of London I wouldn't have explored on my own.

I also went on one of "Windsor Castle and Eton", but it looks like that's no longer being offered

Website: http://www.walks.com/

Cost £6.00

London is generally much safer, even in the bad areas, than comparable US cities, since you don't have to worry about everyone having a gun in their backpack.

Unfortunately the crimes against persons rate has skyrocketed in London (and other European cities) over the past few years. Robbery, assault, home invasions, muggings, etc. are at higher rates in London than in New York. However, the good news is that the vast majority of these crimes are not committed where and when tourists will usually be found. But you do need to be aware of your surroundings.

If you do run into trouble, immediately report any crime to the Metropolitan Police.  The emergency number is 999.  Their website has some helpful information for tourists: www.met.police.uk

THINGS TO DO

Exploring

Speakers Corner at Hyde Park on a Sunday, usually towards afternoon. Fantastic free entertainment! Any London park is a wonderful place to hang out.

walk down Regent Street to Piccadilly Circus and on to Trafalgar Square; 

changing of the guard at Buckingham Palace;

The British Museum is a short walk away from either Oxford Street where you can do some shopping, or alternatively you could walk down Charring Cross and check out the book shops and the theatres, and end up at Trafalgar Square and the Portrait Gallery.  

Another good day would be doing the river walk on the south bank, from which you can access the Tate Modern and the Globe.

The Big Five

Also, I really like the British Museum and especially the Victoria and Albert. You'll be over there when the gardens will be glorious.

The British Museum is fantastic for archaeology,

Castles

Have you been to Hampton Court or the Tower of London?
People talk about how touristy they are, but there is a lot of history there, and those Beefeater guys do a great job. 

We also did a day trip to Hampton Court by ourselves and enjoyed it very much. Hope this gives you some more ideas for your trip.  

We went to Hampton court and really enjoyed it, the gardens were so pretty! We did a bunch of the typical tourist stuff. We were there for the Queens Jubilee and that was fun. I like the tea idea. I'm not a big fan of tea but scones...

Have you yet been to Hampton Court Palace? London Walks has an all-day tour there that is a lot of fun.  

Museums

Sir John Soanes Museum. The Soanes is pretty cool because it was a man's home, it has hidden panels and an incredible collection of artifacts, and it is not as overwhelming as some London museums.
Soanes

I was blown away by the treasures of the British Library and loved looking at the manuscripts and documents.

A fabulous museum is the Courtauld Institute. They have Manet's Bar at the Folies-Bergere. I'd go to London for the day just to see that.  

Have you thought about going to Greenwich to see the National Maritime Museum and the Royal Observatory? I really enjoyed both of those.

The Cabinet War Rooms might be worth going to visit. It is an underground bunker in Whitehall that Churchill used during WW2. Wrong war I know,but still interesting. I went there a couple of years ago. They still have the old transatlantic telegraph machine that was used by Churchill to chit chat with Roosevelt

It's just behind Downing Street off Whitehall or you can get to it through St James Park(The Queen's front Garden)

Or put another way it's about 5/10 mins walk from Trafalgar Square heading towards Big Ben

CWR

More interesting war stuff can be found at the Imperial War Museum in London - in (or near) Pimlico.

Art

The Tate Modern This place was mind-boggling. Well worth a visit.  I've been to the Tate Britain. They have some cool stuff. A Pre-Raphaelite show begins there next month but I think it ends May 3

National Gallery

Churches

Westminster Abbey

St. Paul's Cathedral 

Markets

London Street Markets I've been to Camden, Bermondsey and Portobello. Found a wonderful antique pin at Bermondsey, and I am always looking for old postcards. There's Covent Garden, too.
 

Covent Garden on a Saturday morning was fun with the market and street performers.

   

My favorite London things to do are
--seeing a show
--walking around the markets
--shopping around oxford circus, regent street and also covent garden (which had soo many new stores the last time i was there!)
--going out at night in Soho, near Tottenham Court Rd
--exploring the bookshops (Foyle's especially)
--National Gallery of Art and Tate Modern (gorgeous!)
P.S. nearly forgot - I always scour Time Out as well and take advantage of live music

Most of the museums - British Museum, British Library, National Gallery, the Tate, the Tate Modern and National Portrait Gallery have free entrance and free guided tours.


Other attractions like the Tower, Hampton Court are kind of pricey to enter, but if you are a history buff it's well-worth the price. I took a London Walks explorer day to Hampton and it was expensive but worth the price.

London is very exciting at Christmas time (although I flew out on the 23d of December). The markets and shops are festive and churches have free choral programs. You'll have a lovely time.

Stay in London and take day trips. The Christmasy atmosphere is everywhere. Others might disagree with me, but if it is the first trip for you guys,I think Hampton Court and The Tower are a must (or at least one of those). At the Tower, one of the Beefeaters looked at the crestfallen look on my spouse's face upon hearing the price, and they waved us in for free. Those guys give one heck of a tour, as well.

   
And I am usually not much for modern art, but the Tate Modern was fab. Another way to spend a couple hours is the Soane's Museum, which was actually a man's home (full of British Empire plunder.)Soane's Museum
Hopefully Lost Sheep and others will kick in with some advice.

 I also second the recommendations on London Walks, the beefeater tours at the Tower of London, and Hampton Court. Something else you might like is the Treasures at the British Library. Their collection blew me away

The V&A is free too. For the tower, lines could be long at times. Get your ticket at a tube station to avoid the lines.

London certainly has enough to see for 10 days. Nevertheless, while I like London, I feel a sense of relief to get out of it. London is specially good in having large park areas within its bounds, esp. Hampstead Heath, Greenwich Park, and (on the edge of London) Richmond Park. Any of these give a nice half-day break. (But if you want some real open country and fresh air, come to Shropshire!).

You could check out the horrendously expensive and ridiculous London Dungeon, and/or Madame T's. Shakespeare's Globe is worth a visit.

But all of the museums, including and especially the British Library, are enthralling.
One of my favorite things to do in London is visit all the flea markets. They have the most incredible stuff. And Portebello Rd on Saturday is overwhelming.

I'd like to recommend the Science museum - it has a really cool section for kids called the Launch Pad, I have great memories of finding out how things worked there. It's got some great stuff for grown ups too.

Also, go to the Natural History Museum. The architecture of the building itself is fantastic and you'll never see the whole thing in one day, but check out the full sized blue whale that hangs above the ceiling - it's huge.

Both of the above are free like most museums in London. Also, go onto the London Eye, great views even when it is grey and wet and it's relatively cheap (about £12.50 I think).

I loved Hampton Court it's not hard to get to. I also liked the Tower of London. Lots of cool museums and galleries.

I used to go thru Victoria Station quite a bit. I loved (and still love) to visit the Dorothy Perkins in Victoria Centre. Close to all the theatres.

I'd like to recommend the Science museum I love that place

They have an original Apollo Space Capsule in there and a Yellow McClaren F1

Natural History is a good call too

I think it's better than the one in NYC, except they don't have that big blue whale hanging from the ceiling like in NY