Another way to go is by river, via the Thames Clipper ferryboats.
Tap in with your Oyster card when you get on; tap out with the Oyster card when you exit the boat.
Route map here:
https://www.thamesclippers.com/plan-your-journey/route-map
The Thames Clipper is an easy way to get around London without using the tube.
You can get on at Westminster pier, and get off at the Tower of London pier.
Or continue on to the east to Canary Wharf and/or Greenwich.
It's fun to see the London landmarks from the river.
The Museum of London Docklands is excellent, and currently has an exhibit about Mudlarks of London and their finds. Mudlarks are people who walk the shoreline of the Thames River at low tide, looking for relics of the past, such as Roman or Tudor coins, rings, pipe stems and more. Some of their finds are currently on display. Most interesting.
The Museum of London Docklands is part of the Museum of London, which is also well worth a visit. They have lots of Roman Britain finds, including complete mosaic tile floors from some Roman townhouses found in London. The bombing of London during WW2 destroyed many buildings, but revealed many remains of Roman buildings that had been under their foundations. Many of these finds were taken to the Museum of London and put on display. They have a good cafe for lunch and refreshments as well as a good bookstore/gift shop.