Sunday evening

Don’t bother with the Jack the Ripper tour because none of the original buildings exist. There is an evening hop on hop off bus tour. You could also go to the theater.

Doesn't London Walks have a pub crawl?

It will be dark by 4pm.

Check out the Christmas lights on Regent and Oxford streets.

Walk around your neighborhood.

Walk along the Thames River Path.

Walk across Tower Bridge.

Some evening options off the top of my head: London by night bus tour (starts near the Ritz, Piccadilly), theatre and dinner, Ceremony of the Keys at the Tower of London.

If the weather is good, you can just go for a stroll in one of the many London parks or along the Thames path. This is what I personally do after I land in London late afternoons, because I want to keep moving and be outside after my long flight.

Have a great trip!

Cynthia, I wouldn't pre-book anything at all for arrival day. You might be dealing with jet lag, and there's always the possibility of a flight delay. At most, I'd find a self-guided walk (provided in any number of guide books) to do at your own pace, an early dinner, and a bedtime that will allow you to re-set your internal clock to London time on Day 2.

We're all different but I'd personally avoid any sort of vehicle tour like a HOHO bus 'cause riding around with jet lag would put me right to sleep. YMMV. We landed into London in the morning some years ago, and found coffee + walking on our own until supper what worked for us.

At Heathrow right now and here are a few things to remember.
1. Trains on strike
2. Weather is impacting flights with cancellations and delays
3. E gates make passport control quick and fairly painless.

I would hold off on planning too much structured activities until you see how your actual transit day goes. I do like the walking/Christmas light idea.

When we arrive after the long flight, we get into our hotel and take a nap.

Then we go out for a bite to eat and take it easy the rest of the night. We wake up next morning ready to hit the town.

Cynthia, after a long period of bitter but sunny cold, we are promised somewhat warmer weather from this Sunday evening, but rain during Sunday daytime, so I’d wait to see what you feel like doing. The suggestion about seeing the. Christmas lights is great, walking round the parks or the Thames pathway is not, as it will:be getting dark by 3.30ish and dark by 4pm and the parks close then. However, museums and shops will still be open and it will be lively as people will still be out doing Christmas shopping etc.

There are a couple of pieces of information we need to give a good answer:

1) In what area of London are you staying?

2) Where are you flying in from? The answers you get will differ if you might or might not have to deal with possible jet lag.

Fortunately for you, your arrival date is known as a day of miracles in London--no one is on strike.

If you are arriving from North America, I agree, don't make plans ahead of time. You don't know if your flight will be delayed or if jet lag might hit you.

I should have mentioned that the trip is planned in mid-March so we'll have a little more sunlight, but we will miss the wonderful holiday decorations!

Thank you for your ideas. We are staying between Hyde Park and Paddington Station since we are taking the Heathrow Express into town and got the discounted tickets for advance purchase.

I believe that most of the theatre productions are off on Sunday evenings, but I will double check.

Definitely wrong about theatre.
https://www.londontheatre.co.uk/whats-on/sunday-shows

Have dinner at Victoria Paddington Pub or at Tudkin or tube to Westminster and see Parliament Square. Then walk up Whitehall to Trafalgar Square. Snap pics then head to Chinatown to dine at King Joy Lau.

In other words absorb London by exploring on foot.

I believe that most of the theatre productions are off on Sunday
evenings, but I will double check.

Even if they are running, I sure wouldn't do this on an arrival day right off a transatlantic flight, if indeed that's what you'll be doing? As said before, we're all different but I sure wouldn't plan anything at all that demanded my full attention, especially if it involved having to sit for some hours in a dark-ish theater after all those hours sitting on a plane. I'd probably go right to sleep!