If it is a nice day, you can walk down from St Pancras to Trafalgar Square and on to Westminster Abbey and Parliament. Maybe 2 miles. Or the number 91 bus from St Pancras to Trafalgar Square and walk the balance. http://tinyurl.com/cgbuqd (Made from the London tube website journey planner which gives you very specific map to find bus stop if you click on pdf map icon for the first step.)

I like KayF's suggestions of The Eye, Tower from the outside, and Regents Park/Little Venice.

If you cross Tower Bridge to the south bank you'll find a range of places to eat in the Butlers Wharf area.

I don't think KayF meant all those things - since they are in three entirely different parts of the city. I'd guess she was simply making suggestions for some options.

But do I have to disagree a bit about a 'walk-by' of the Tower. From the outside you really don't see much of anything except part of the White Tower sticking up in the center. If one doesn't have time to go inside, there are other sites which do give good views.

Walking along the southbank w/ views of the north bank including Tower Bridge, St Paul's, houses of Parliament, Big Ben, plus the Globe and Tate Modern. Of course, one can start from Tower Hill tube station, see a glimpse of the Tower, cross the bridge and then the length of the southbank.

Or walk from Buckingham Palace down the Mall to Trafalgar Square and then - down Whitehall - or to Covent Garden - or over to Somerset House

You can easily do all three in a day by using the tube, and I think if you only have a day, picking a few outdoor sites (as opposed to shopping and museums, which are the other main suggestions on this thread) will give you more a feel of the city.

I also think the river area (whether Tower Hill, Westminster or the South Bank) is a lot more worthwhile than Piccadily and Trafalger Square.

I would start on any initial visit to London that folks start at the London Eye Ferris Wheel as this gives you a bird's eye view of all the famous landmarks and the lay of the land - and it is centrally located to walk by all the places Janis mentions and which i also endorse as a great stroll.

And yes your view of The dread Tower of London from the Eye over London is all you need if not going in - unfortunately going in involves usually a long wait and amount of time once inside.