We took this walk in the late afternoon on a weekday, so the hustle-and-bustle of London was lots of fun to watch.
Start at Westminster Bridge, enjoying the view of Big Ben and the Houses of Parliament from the middle of the bridge, and then walk back towards the city. The view up and down the river from here is gorgeous as well.
At Parliament Square, you can also get a glimpse of the two white towers of the beautiful Westminster Abbey (definitely worth a visit!) In Parliament Square, there's a famous statue of Winston Churchill and other important dignitaries.
As you start your stroll down Whitehall, you will see all the most important buildings in London--ministries of finance, treasury, etc. Stop at the Cenotaph, a beautiful monument honoring the dead of WWI and WWII. Depending on the level of security, you can also peek down Downing Street to see the prime minister's residence. You can also see the horse guards a little farther down.
As you walk, look occasionally back towards Big Ben. The views up and down this street give continually great photo ops.
We stopped for beer and fish-and-chips at Clarence Pub along the way. Delicious!
Soon you'll be at Trafalgar Square, where you can climb the lions, feed the pigeons, and people-watch. Soak it in!
If you're not too tired, you can head down up to Piccadilly Circus, the Times Square of London. We walked down Piccadilly toward Green Park back to our hotel. There were many opportunities to buys souvenirs or stop for a snack.