Haven’t lived in either city in a long time so my “local” knowledge is outdated (@AFJF can help you with London I’m sure) but I do have to say 1 of my 2 all time favorite “tourist traps” in the world is the Tower of London. I ******* love it. The full tour. Torture chambers, tower where Queen Elizabeth I was held prisoner, hall of armor, royal bedroom, royal crapper (a hole in a board that leads to a hole through the wall), Crown Jewels… cannot get enough. Every time I’m in London I go. I don’t care that I’ve been there 25 times. And I been everywhere. Lost track of how many countries but could well be over 100 at this point. I also dig the Jack the Ripper night walk, which is close to Tower of London. If you do that, at the end, ask the tour guide if you can buy him a pint afterward and he’ll give you even more interesting stories over an ale. I talked to mine for an hour about his theories of who the ripper was.
Paris — I can say Mona Lisa is over rated (IMHO) but you almost are obligated to go. I preferred Musée D’Orsay to the Louvre. Van Gogh’s work in person? Wow. Not an art guy but was a religious experience for me. The brushstrokes add a third dimension that you can’t see in photos. It was surreal and I could not turn away from a few of them. Especially his haystacks series.
PS: my 2nd favorite tourist trap is the DMZ between N. And S. Korea. When my oldest daughter lived alone with me in Asia, I took her with me on a business trip to Seoul. My ex made me promise not to take her “into North Korea.” Yeah, sure, whatever. I booked a tour and I took her up there. “Can I sneak a picture and put it on my Instagram, dad?” Yeah go ahead. 30 seconds later I get an incoming call from Philadelphia lol.