as others have said The Duck Tour is a must, The London Eye, all in the same area, Southbank has lots of cafe's, restaurants for a quick stop.
Natural History museum is wonderful, gets quite crowded, especially as it's half term. Covent garden is quite interesting, they usually have musicians, sometime jugglers, and shops to browse. If you want to whizz around the sights, like buck house, and tower of london, take the open top tour buses,
too many things really, some off the track i could recommend but not sure if you have the time, or will find them. If you are not taking in a tour bus to see the sights, then get oyster cards for public transport, see around London that way. lots of choice, have a good time.