The third day of your 2-day itinerary can be Museum day. If you are not fond of museums, then have a look at other itineraries which can be managed in a day.
London Itinerary for 3 days
2 day England Itinerary can include Museums. See the main museum map in London
There are several museums in London which cater to all ages. From Sherlock Holmes fans to Harry Potter fans.
National Gallery Museum
A good place to start your historical journey is Natural History Museum / Science Museum (Nearest subway station is South Kensington) which are next to each other. These would be liked by children and adults alike. The highlight is the huge Dinosaur skeleton at the entrance. You can also see an IMAX film in the Science Museum. The best part is that the museums are free entry.
Although above National gallery museum is not on this itinerary, you could visit that in Trafalgar square (see day 1 itinerary). It does have few famous artifacts to see including paintings.
You have to pay for the IMAX. Visitors from the commonwealth may enjoy the next door Victoria & Albert museum. This would take a whole day. Another top class museum is British Museum. For modern art lovers, you can choose to go to the Tate gallery. After museums, if you still have the energy left, you can go to the Leicester square/oxford circus area to relax.