I am taking DS1 (6yo) to London in a couple of weekends' time and am looking for something to do on the Saturday afternoon for a couple of hours. We have done the Science Museum before so I was thinking of maybe the British Museum, Nat Hist Museum, Greenwich planetarium or the Horniman Museum (we are getting the train to central London but staying with friends in SE London).
Any recommendations as to which is best? I was leaning towards the planetarium but have never been (haven't visited the others in years). He loves all things sciency and hands-on stuff.