OK...where would you go for an afternoon to sightsee in London? I do not know much about London. We will have an afternoon and an evening both going and coming from the US. Much appreciate any info on this!

Since no-one else has come up with any suggestions, I might as well try.

First of all, how much time will you really have if you're between planes? Will your luggage be checked right through? Depending on the time of day you arrive, it could easily take an hour to get through immigration to be ready to leave the airport, and then you have to allow for whatever time you have to check in for your next flight (and check this with the airline).

Second, assuming the time left is more than 3-4 hours, or so, what sort of things interest you?

One option people suggest - and this is probably the only practicable option if you do only have about 3-4 hours - is to take a taxi or bus to Windsor, which is nearer Heathrow than central London, and is an attractive town in its own right. You can see at least the outside of the Castle, and have a meal and a stroll by the river.

To go into London, the quickest - but not cheap way - is the Heathrow Express (15 minutes to Paddington). From there you could take buses or the underground to whatever you want to see. Get a one-day Travelcard for zones 1 and 2, and that covers all buses and trains in thezones, and gets you

Clicked too soon!

...gets you one-third off boat rides, if that interests you - might be a possibility if there's time.

From Paddington, you could take the number 15 bus all the way through central London to the Tower. Much cheaper than the hop-on hop-off tourist buses, though they would be more thorough and would give you a commentary.

If you haven't much time to get the best out of the paid-for attractions, it seems to me sensible to see at least a bit of the free ones like the National Gallery or the British Museum (see www.tfl.gov.uk and look for the visitor maps).

If you're using Gatwick airport, then trains to central London take about half an hour, or you have the option of going the other way, to Brighton by the sea (also half an hour).

Hi Patrick...thanks for answering. We will arrive at Heathrow at noon and leave the next morning out of Gatwick. On the return we will come into Heathrow at 3:30 pm and leave the next day at 3pm out of Heathrow. thanks for the suggestions. my husband has been to London before but he was very young and does not remember much....hey, he cann't remember what he had for lunch yesterday! LOL...

In that case, I would recommend the hop on hop off tour buses. The commentary is worth it to give you some ideas of what you'd like to actually see and to orient yourself in the city.

In some ways it would make sense to do this on your first afternoon because you could then identify things you want to come back to explore on your homeward leg. But you'll have more time on the homeward leg to make full use of the hop on hop off capability. On your first journey I doubt you'll have much time to take advantage of it.

If your free time is short at the airport--let's say 6 or so hours at Heathrow--you could consider taking a taxi (or bus) to Windsor, which is fairly nearby.

But I agree, that your actual free time may be less than it appears for your layover once you subtract security clearance, baggage, checkin, etc.

If the weather is nice, I suggest going for a ride on the London Eye - you will get some fabulous views!

My advice is to head for Central London area. I remember spending one great morning starting a walk in Convent Gardens, near Charing Cross. Here you can see something of real London. There are markets, restaurants, pubs, as well as nearby St Paul's and the theatre district.
Then find your way to Trafalger Square and rub elbows with a gazillion tourists. Right here are the National Museum, the National Portrait Museum and St Martins in the Field. Then I took a stroll down the Mall to Buckingham palace. Here I ran into horse mounted Redcoated Royal Guard Squadron marching from / to the Palace. And a little countryside in the City (St. James Park). Then to Westminster and the houses of Parliament (and another gazillion tourists), the River Thames (boat tours available), (the Ferris Wheel), and Westminister Town Hall. While walking I see the Royal Horse Guards parade grounds up a back street and enjoy an impromptu horse mounted military ceremony.
Then I returned to Convent Gardens for lunch.
The whole walk was long but took only about five hours of my life and an experience I'll never forget.
I didn't go to London ... I let London come to me all in a little more than one-half day.

Thanks everyone! We ended up on the out bound staying by Gatwick and going to Brighton for the late afternoon/evening. We lucked out and had great weather!! On the return we stayed right by Trafalgar Sguare and i think between the evening and the next day we walked just about everything you could! Again, i thank you