Places to visit in London with dd aged 4

[EleanorRugby]

My youngest dd starts school in September so I am planning to make the most of our last few weeks of being able to visit attractions during term time and visit some places in London on Fridays when we both have the day off work/nursery.
We live in SE London so can easily get train into Victoria, London Bridge or Charing Cross but I need to be back for school pick up for dd1.

Places we have already visited and enjoyed are:
Natural History Museum
London Transport Museum
Museum of Childhood
Discover Stratford
Horniman Museum

On my list are Sealife Centre (with Kellogs free adult voucher!) and Diana playground.

Does anyone else have any suggestions for free or relatively cheap places to visit?

Thank you

science museum

Museum of London

Cutty Sark and Maritime Museum, the first costs money, the second is free.

Even when your DD doesn't pay for the train you can still use the 2for1 offers if she needs to pay admission fee, I did this at Sea Life.

Tower of London?

Coram's fields near the British Museum

Queen Elizabeth Olympic Park, with a swim at the Acquatics centre