I would second the Imperial War Museum, we went to the one in Manchester and plan doing the London one at Easter. HMS Belfast is also good, I believe you can also go in Tower Bridge, and check on the website for the times at which it will be opening. Walk along the south bank and peer through the gate at the Globe Theatre. The London Eye is also great, we'll be doing this for the 3 time, each time we have just turned up and jumped on. The first time I took my son to London he was 4 and got lots of thrills riding at the front of a double decker bus and watching and riding the tubes.... Chinatown and environs is a great place to eat, and often the museums have good cafes. Greenwich is another place that may be worth a visit, there's the maritime museum, market, park, and Cutty Sark, you can take the DLR to Island Gardens (? End of the line) and walk through the tunnel under the river to Greenwich. (theres also a very large very new Lego shop in Leicester Square) St Martin in the Fields used to offer brass rubbing in the crypt for a small fee, not sure if it still happens. The Natural History museum is fun, and free.