I've taken tour busses on the first day I arrive (London was really good to see that way, IMO) Then follow with up by car with a sightseeing drive to see things the tour bus might have missed and/or a closer look at what you like. Lot's to see in both but I've been to London several times and still find things I've missed and that I find interesting. YMMV.
There are good guided tours in both cities.
Some of them are "free" that is you're invited to tip them after a couple of hours of walking around the main sites.
Even the free tours require booking though.
This is an institution in London.
https://www.walks.com
Been a long time since I've been in museums, was losing interest even before the pandemic and haven't thought of visiting them now.
But if the OP is interested in museums, most of the London museums are free, the major ones. I've never seen big queues either so you can walk in, wonder about.
But Paris museums are a different story, they are often packed and the queues to get into the Louvre, the Orsay and a few others can be bad during peak season.
One option is to get a Museum Pass, usually good for 3 or 4 days. They are also offered with metro passes for the same period. But for the Paris Metro, I just buy carnet or a 10-pack of individual tickets and I don't use them all. However I read recently they may be doing away with them and forcing you to buy and load contactless cards of some kind.
Good thing about the Paris Museum Pass is that you can get into shorter queues than the main ones. They may call it skip the line but there's just shorter queues.
The trick is to go to a smaller museum to buy the Museum Pass. I've gone to Invalides (Napoleon's tomb) or the Conciergerie to buy the Museum Pass before hitting the bigger museum.
Also, you used to be able to enter the Louvre from a separate wing, away from the main entrance through one of the pyramids in the main Cour. You also used to be able to enter through little underground mall leading to the Louvre from the Palais Royal metro station. It's called the Carrousel du Louvre. But I think you must already have tickets, you can't get to the main ticket office queue there.
Getting into the Orsay is a pain, they search your bags and they used to let you take pictures everywhere -- it's a very beautiful space, even apart from the art -- but they prohibited that so I stopped going.
That's the other thing, be prepared to put most of your belongings in cloak rooms or lockers. Have some 1 or 2 Euro coins handy for the lockers.
In London, there is a bus line that goes through most of the attractions so instead of paying for a tourist bus, you can board one of these for awhile.
https://www.tripsavvy.com/london-bus-routes-for-sightseeing-1582177
Some buses, the upper deck is usually the first place people go so there may not always be space up there but obviously best for seeing the sights.
If you have Apple Pay or Google Pay on your devices, you can tap in to board buses and metro trains. As long as you use the same device and card, I think there's a daily limit of like £16 or something like that. Alternatively you buy an Oyster card and load it up but you may not use up all the money you load and getting refund for the unused portion is a pain or not even possible.