Hamley's. It's one of my favourite places in London, I love their displays and love just wandering around the masses of toys.

I could also spend way more than 2 hours in the National Gallery, have you thought about the British Museum, the Tate, Natural History Museum etc?
Also, the tours of Globe are really interesting, I've yet to actually see a play at the Globe but hope to this spring/summer.
If you can't catch a play at the Globe, you MUST watch something at one of the West End theatres. And a trip to Haagen Dazs in Leicster Square is always a must for me.
Sorry if that seems a little rambled, just got home from work and my brain has become mushy mashed potatoe!

As you can probably tell I love London, I'd go there every week if I could afford to.