If DD likes history but thinks the popular London historic sites might be too much then I have two alternatives, Fulham Palace ( Chelsea way) which is free entry or Eltham Palace (south London but easy bus ride ) If you have or can borrow an English Heritage or Art Fund card from someone it will cut down costs. Of the two I prefer Eltham but they are both good for a visit.
I am not sure why you say no boat trips, the trip down to Greenwich on the normal passenger boat is lovely, and very interesting as you get a different perspective on the riverside buildings, eg the Tower of London, HMS Belfast, Tower Bridge, a sight of Canary Wharf, St Katherine s Dock - Greenwich is a great place to visit, with smaller crowds than many central London places and lots to see and do.
if you are in London on a Sunday morning then a trip to the flower market in Columbia Road is a busy but interesting visit, follow up with a mooch around Spitalfields Market.
For quirky search out Postman’s Park, Sir John Soane Museum, The Foundling Museum( look for the Tracy Emin work on the railings) , Corams Fields for a breathing Space.
The British Museum is always busy, but it is huge so there are always corners to explore.
I am not sure of the age restrictions but check out the London Transport Museum to see if they are doing any of their amazing abandoned tube station visits, run by enthusiasts they give you a very different perspective of London and its development.
Take a trip to Stratford on the Elizabeth line and visit the Elizabeth Park, formerly the Olympic site ( 12 years ago, doesn’t seem possible) . Great walks, gardens, buildings, take a canal trip, hire a bike, lots of places to eat.
Kew gardens has been mentioned a few times, it is eye wateringly expensive, but worth it if you grit your teeth. I would opt for the Chelsea Physic Garden, smaller and cheaper, but have lunch in the Army Museum just up the road, better and cheaper food than CP and often very interesting displays and exhibitions, not just about army life.