20 hour layover in London

Windsor is the obvious choice. The easiest way is by taxi. However, the issue is that you are arriving on a Sunday - by law, shops can only open for 6 hours and most opt for 10-4, so by the time you are through the airport, the shops will have closed. Most pubs don’t serve food on a Sunday evening, but Windsor has plenty of restaurants that will be open.

You won’t have time to see anything on the Monday morning.

Just for consideration: Gate checked luggage on most airlines these days require you to go to baggage claim to retrieve it. They do not return it to you when you exit the plane, with the exception of mobility devices (wheelchairs, baby buggies and similar) or on small regional jets (which I doubt you will be on for this trip). Please consider this when calculating your free time.

Thank you both very much. On second thought, I'm wondering whether because it is Sunday, and perhaps less traffic, I should go on into London rather than Windsor? Thoughts on that? And a question about checking baggage at the gate. I've only recently begun doing this. I've done it twice in domestic (U.S.) travel. I was surprised how easy it was and how they had the bag waiting for me just as I exited the plane. Was that just a fluke? Or, is international travel different in that way. If gate-checked bags have to also go through the baggage claim, should I just check it in ahead or time anyway? Or, chance that I might be allowed to gate-check it and hope that it will be available to me as I exit the plane? Many thanks.

It sounds like your US to Heathrow flight is an overnight flight? I'd suggest just staying at the Sofitel or other airport hotel and just have a lovely dinner. Chill out in your room or the bar, and go to sleep around 9:00 or so. You will likely be exhausted with little sleep on the flight. I like to arrive healthy and rested at my destinations. I arrive at Heathrow about 7-8:00 PM on a day flight from the US, and stay in the same terminal as my flight in and out, at the Sofitel. It is more important to me that I don't get sick on my vacation and that I enjoy my entire transport time, rather than starting out exhausted. This is just a slightly different idea than others, but I thought I'd put it out there to give an alternative to consider.

With just a few hours, you won't see much in London or Windsor (more in Windsor), and if London then mostly Tube. You need to decide if it is worth it.

Also, if you think they might check your bag at the gate, you might as well send it originally as checked baggage so you can make sure it is properly label when it leaves your hand. The flights I've had internationally, the baggage is sent to baggage claim. I check my carryon most of the time so I don't need to have it with me in the secure area of the airport. Properly labeled, I have had no problems over many decades.

Wray, thank you so much for your thoughts.

You can also visit Legoland if you are into Legos and if you will go to Windsor.