Ideas for overnight layover in London?

Is your flight the next day at Heathrow, Gatwick, or Stansted?

You could still do the bus tour of London. The Hop-On-Hop-Off bus does a 2 hour loop around London, passing all the sights. Then it does the exact same loop, again and again. You could still enjoy the ride for the 2 hours. Problem is, you will be paying for a 24-hour ticket, which is how long most people use it for, using it to hop off and see sights after they have enjoyed the "tour" of the first 2 hours. Still, it's a good use of your time to see, as in pass by, the maximum number of sights quickly.

A good neighborhood to stay in? It depends on how much time you will have to stay in London the next day before dashing to the airport. What time does your flight depart? From which airport does your flight depart? If you are leaving early the next day, to go to Heathrow, I recommend the Paddington Hilton, connected by a walkway to the Paddington Station, where you can catch the Heathrow Express or Heathrow Connect trains.

You do not say exactly what time you will get in on Saturday afternoon/evening. If it is early enough, I would see the Tower of London or Westminster Abbey. You could go to the late-night Ceremony of the Keys at the Tower of London, but you'd best apply for (free) tickets as soon as possible. They go fast. It's up to you if you'd rather see a show. But your sister, who has never been to London before, will probably want to see some of London.

We fly into and out of Heathrow. The arrival time is 3:30 pm Saturday and we fly out 9:30 am Sunday. I was estimating 2-3 hours to get out of the airport and into a hotel. I haven't looked at all the bus companies, but the ones I have seen it looks like they stop running about 5:30 pm. I wish they were still running, because I wouldn't mind seeing the city that way at night, but I don't think that will be an option. I think we'd be OK going until late that night, if there is something for us to do. I figure we can sleep on the plane--not that I'm looking to pull an all-nighter.

You will be getting into town just as the Tower Of London is closing, so scratch that. You can take one of the city buses that travel by the tourist sites, like the number 15 that runs from Trafalgar Square to Tower Hill, just to sightsee as it passes some of the tourist sites.

Best use of your time is to take a walk around Westminster Abbey, Parliament, Big Ben, then cross Westminster Bridge to the South Bank. Walk east along the riverfront past the London Eye, Shakespeare's Globe Theatre, and continue on east to Tower Bridge. If you walk across the bridge, then you can go and stand in front of the Tower of London and get a good look at it before continuing your walk.

Your flight out in the morning, because you should be there no less than two hours early, makes me think you might want to stay at one of the airport hotels. You could arrive at Heathrow, check into one of the Hiltons at either T4 or T5. Leave your luggage at the hotel. Then train or tube into London. When you've seen a bit of London, take the train or tube back out to your Heathrow hotel to sleep. Makes it much easier to be on time for your flight the next morning.

If you were determined to spend the night in London, you may want to stay at a hotel near Paddington Station. If you come into town on the tube (Piccadilly Line from Heathrow) it takes you right to several good neighborhoods. South Kensington, Leicester Square, Russell Square are all good locations. These are good locations in which to look for a hotel or B&B. Look at a tourist map and decide if these locations appeal to you.

As far as seeing London neighborhoods, look at the website for London Walks Company. Guided walking tours of many different areas of London for not much money. www.walks.com. It would be a fun evening to go on one of their walks. The guides tell you all kinds of history and details about London. Have a look at their walks.

You may want to purchase Rick Steves Pocket London Guidebook, sold here on this website.
https://store.ricksteves.com/shop/p/pocket-london

That sounds like a great plan Rebecca. As I thought about it, I did think staying in an airport hotel would be easiest.

There are several decent and reasonably priced hotels just outside Paddington Station – the Darlington comes to mind. With the Heathrow Connect (~45 mins) and Heathrow Express (~30 mins), Paddington would be my choice over an airport hotel. You could be checked in by supper time and have a nice walk along the Esplanade, maybe fly the Eye, or take an evening Thames sightseeing cruise. Much (much - MUCH) nicer ambiance than the airport.