One day in London

Rick's Paris guidebook has an entire chapter dedicated to doing London day trip efficiently. In general, the Eurostar arrives at Waterloo Station. Waterloo is only a block away from the London Eye, Big Ben is just across the River Thames. From there it's a short walk to Westminster Abbey and Buckingham Palace. All the other major attraction, depending on your interests, are easy to get to via the Underground or one of the hop-on-hop-off tour buses.

We are also spending just one day in London when we tour Europe, so I too have been pondering the best way to utilize our short time there. I agree with the previous poster. Rick Steves states that all of the "visitor's London" is located between Hyde Park and The Tower of London. He says this is a great three mile walk. He even has a map in Europe Through the Back Door. We are planning to take the walk and just see what appeals. We really are not that interested in dropping GBPs left and right to see the Crown Jewels, etc. etc. We're more interested in just seeing the sights and enjoying the ambiance. We have also considered hopping a local bus that covers the same area, but am not familiar with the bus routes. Also there are the Big Double Decker Bus Tours, if you're so inclined. www.bigbustours.com But, in my opinion, the walk seems like a better way to really experience the city.

The Transport for London site, www.tfl.gov.uk, has lots of helpful information. Here is a link for printing a two-page map: http://tinyurl.com/25p633. One page is a tube and train map and the other page is a central London bus map that also shows the location of major tourist attractions.

We had just 1 full day in London last year as part of a longer Scotland trip... We decided to do the cliche double decker bus tour. It was a great way to get your bearings. Buses allow you to hop on and off as you please and spend as much or as little time in each stop as you need. We were able to see the sites we wanted, spent time at Buckingham Palace, Westminster Abbey, Lunched in the park, a little time at the museum and early dinner at a pub. It was a nice overview of the city. I didn't feel like I missed out only having 1 day. It let me know what I want to see more of next time!

If you've only got one day use a double decker bus and see as much of all the famous places that you can. Don't forget Trafalgar Square and St. Pauls. When you're weary and need lunch, if the weather is good, have lunch in the open air at Covent Garden. You can always try a bit of 'Celeb Spotting' while you dine. There are lots of big name theatres close at hand and well known actors and actresses walk through Covent Garden. I spotted Kevin Spacey once.

I agree with the post from Al. You should see Trafalgar Square and St. Pauls. While you are at Trafalgar Square walk across the stree and go inside of St. Martins-in-the=Field, it's a beautiful church.
Covent Garden is a fantistic place the get something to eat. Have a great trip.

Hop-on-hop off gives you a great overview and you won't miss anything "big" (really, you must see the Parliament building, even if it's a drive by). A friendI loved St. Paul's more than Westminster. I would also pop into the British Museum (free) and hit the major highlights (Rosetta stone, mummies, etc.). Or do the same at the National Gallery (also free) to see Van Gogh's "Sunflowers" and Seurat's "Bathers". You'll never see it all so just focus on something you like. Take the earliest train you can on the way and the latest on the way back. The Waterloo station has a decent Sainsbury's (or the like) with portable food for the return trip.

I agree the hop on hop off bus is good to get a view of pretty much the entire city, but only if you are on the upper open deck, and it should be nice weather. Then you can jump off whenever something strikes your fancy or just sit.
If you don't want such a blatently touristy thing though, I very much agree that the Westminster area with the Abbey, Parliament is a great area. From there you can walk to Buckingham palace, Trafalgar square and even up into the West End and Covent Garden. That should more than take up a day. It's not convenient to get to St. Paul's from there, certainly can't walk it. I would choose these other things over it.