DS1 has done all the obvious places in London, but he has an afternoon with Grandad coming up after seeing mummy's office and having lunch with me. I thought this might be an option, or possibly Transport Museum (again) or maybe the Diana Playground if weather is good.
welll,... are you scared of heights or is ds a climber? I took my 2 to the Golden Hinde a few years ago and couldn't get off quick enough. 2 boys with a huge ship in a dry dock, metres to fall with just a small scramble net to stop them <shudders> it was my worst nightmare come true! I recommend leaving that one... HMS Belfast is quite good but tbh I think he might be a bit young to get the full experience. If he likes looking at planes and tanks how about the Imperial War Museum?
Haven't done Golden Hinde, but HMS Belfast is fantastic. Taken DS there twice now - first time he was four. It's absolutely huge too, so rarely gets crowded. They do have vertical ladders, so depends on whether you think Grandad's up for that. Diana Playground's great too if you've an active boy, though it does get busy in the holidays.