One day layover in London

I would suggest staying in Windsor, it's a 15 minute taxi ride from Heathrow. The Royal Adelaide Hotel is very nice and walking distance to everything in Windsor and Eton. Also, there is the Alma Lodge B&B which is nice. If you stayed in Windsor you could visit the castle, walk to Windsor Great Park to see the Long Walk (which is the private road for the Royal Family ti enter and exit the castle). Could walk over to Eton and see Eton College, (which is a middle/high school) where all the upper clase and royals send their sons. Enjoy a great meal at the Horse and Groom Pub across the street from the exit of Windsor Castle. Depending on how long it takes to go thru immigration (could be 20mins to 1 hr), you should be in Windsor by 12:15-12:30pm.

If you arrive at 11am, you could have the afternoon and evening in London. But you need to either stay at Heathrow or leave early the next morning from London.
my favorite area is to walk around Westminster Abbey (visit), Big Ben, Horse Guards, Trafalgar Square; or in that area you could also visit National Gallery.Also all the parks are in that vicinity. Possible to visit theater (if you can stay awake) that night, or just have a good meal. Heathrow departure is a chore, long security lines, so be there the full 3 hours before your departure time. Getting through immigration/customs and to London by tube or train is a lot quicker

I'd suggest staying somewhere along the Piccadilly line. In particular, the area near Earls Court, Gloucester Road, or the South Kensington tube stations is convenient for both getting to and from Heathrow and sightseeing in London. I've stayed at several hotels in this area, but on my last trip stayed at Base2Stay which I really liked for the nice amenities in the room. Arriving at 11 will probably mean that you will not get to London until around 1pm. One afternoon activity and one evening activity (a show or a pub walk) would be feasible. Is your layover coming to the States or returning home? The jet lag situation will impact what is feasible.

I second the suggesstion of Windsor. Makes it easy to get back to the airport the next day. There's lots of good shopping, the castle is wonderful as are its gardens, Eaton is fun and you can even do a short cruise on the Thames (boats board near the bridge over to Eaton). We have stayed at the Castel hotel- nice and not too expensive.

You are likely to arrive in Windsor or London around 1pm, depending on schedules and congestion, etc. But, whatever you do, I like the idea of staying at one of the hotels at Heathrow for an 11am flight. Your trading a late-night trip from London or Windsor for more sleep and and less hassle the next morning.

If you do decide to stay near Heathrow, avoid the Radisson. I stayed there (admittedly, it was more than 5 years ago) and found it to be really expensive (the reservation was made for me.) I wasn't lonely, though. The multitude of COCKROACHES kept me company. (-shiver-)

If you fly from DFW, you'll arrive at Heathrow unless a connection takes you to Gatwick. If your plane is on time, the express train will get you into Central London around 12:30 p.m. either at Paddington Station (from Heathrow) or Victoria Station (from Gatwick). I would stay in London for the night as you have plenty of time to get to the airport the next a.m. If the budget allows, stay around Green Park or Hyde Park perhaps in the Stafford Hotel in the St. James - gaslight district or the Grosvenor Hotel on Park Lane, either of which is about a 10 minute taxi ride from either station. From a central location, you can see a lot in an afternoon and evening. Get a good guidebook and figure out what you want to see.

I think it depends on your stamina, and whether or not you will be jet-lagged. If this is your first-time to London and you really want to see the city, then I would head straight for central London. The area around Trafalgar Square will give you the most bang for your buck because it is walking distance to so many sights--Big Ben, London Eye, Buckingham Palace, Westminster Abbey, Covent Garden, River Thames, theater district. If you're feeling energetic you can easily do a bit of sightseeing. If not, you can sit and chill at Trafalgar Square or on a bench by the river, take some pictures, have dinner, and maybe take in a play. Or take a boat ride (one of my favorite things to do). You could eat at the cafe at the National Gallery or the crypt in the basement at St.Martin in the Fields Church, or go to Covent Garden. This area can be expensive, but you can find discounted hotels at londontown.com and wotif.com. It can also be congested, so you would need to leave by 7:00 or 7:30 a.m. the next morning to be at the airport on time. My favorite hotel in this area is the Strand Palace, mainly for its proximity to so many things. I also like the Royal Horseguards, but it's a bit pricier. A hotel in Westminster would be a good option, too. Good luck!

I agree with Laura, staying along the Piccadilly Line is the thing to do. Most Heathrow hotels are a bother to get to from the airport and a further bother to get into London from. The Heathrow Express saves you no time and costs £12 more each way. If this is your first London visit, consider taking a circuit on a hop on/hop off bus or visiting the one attraction that interests you most. Laura's recommendation of an evening show or pub walk is also perfect. http://www.walks.com Teetotalers have my permission to take a non-pub guided walk. Get up early on departure day and take one of Rick's do it yourself walks before returning to Heathrow.

So many great suggestions! I can't improve on all of this. I will say, take heed of Brian's suggestion, "Heathrow departure is a chore, long security lines, so be there the full 3 hours before your departure time." Agree, totally. Have a great time!

Maybe I've been lucky, but I've never found LHR security queue to take any longer than at any other major airport. If there is any type of "incident" (e.g. weather), it can be crazy, but under normal circumstances it usually seems pretty efficient to me given the size of the airport. Nevertheless, if your airline has online check-in, I recommend you do that the day before you depart. It will save you from waiting in the check-in queue which for some airlines can be worse than the security queue.

If you check in online and only have carry-on luggage, check-in shouldn't take 3 hours. If not, then you might get stuck in a queue.

Thanks for all the great suggestions! This stopover is on our way home, so jetlag will not be a problem.

My experience with the Heathrow Express is different than Chip's. It is a 15-minute ride, while the Tube takes an hour or more. If I was going for the most time-savings for this short visit, regardless of cost, I'd take the Express to Paddington Station and then back again the next morning. Whether I stayed in London or at Heathrow would, frankly, depend on how early I wanted to get up. If you do use the Express, you might consider staying near Paddington.

In my opinion, whether to take the tube or Heathrow Express really depends on where you are staying. If you are staying at Paddington, by all means the Heathrow Express or Connect is going to be much faster. However, if you are staying anywhere other than Paddington, you will need to transfer to the tube or wait in the taxi queue. Much if not all of the time savings can be lost by the transfer and wait. If you are staying along the Piccadilly line, I find the tube to be the better option. One of the reasons that I often stay at Earls Court or Gloucester Road when I have a short stay in London is that it is only a 40 minute tube ride from LHR. Earls Court is only a 15 minute tube ride to Leicester Square.