You might try Greenwich which is my favorite place in the London area.
This is an adventure for the whole family that begins with a boat ride from the Parliament pier along the Thames river to the Greenwich Pier. Disembark and visit the Cutty Sark clipper sailing ship. Climb the hill and see the observatory where you can literally stand over 0 degrees longitude.
This is the home of Greenwich Mean Time. There is also a clock museum which may not sound fun, but it is actually fascinating as it relates to maritime history.
The area includes an interactive museum and a lovely park for picnics. Greenwich is also the home of the Royal Naval College. When I was there visitors were allowed to enter the dramatic chapel and other public rooms free of charge. There is also a marker where Greenwich Palace once was. Greenwich Palace was the birthplace of Queen Elizabeth.