We went to see Sue Maillard last night so I can add a few places for free
Right next to GOSH is Coram's Fields - this has its own website & is a small part for the under 5's.
For the energetic...
With your u/ground zone 1 ticket - go to Covent Garden - this is out only at the moment but then walk down to Chancery Lane / Embankment and cross the river and get a river boat
or then get a bus up to Oxford Circus but hop off towards the top of Regent Street and go into Hamley's - you can escape without spending
From Russell Sq and you could go to the British Museum which has that fantastic new'ish roof (I watched it being built about 6 or 7 years ago)
If you go up river from Embankment it is 70 mins but you can get to Greenwich and go to the Greenwich Conservatory and the Navel Museum -
Don't forget the museum's are mostly free
The London Transport Museum at Covent garden doesn't open till November I think it said yesterday!!
If you do have loads of time then it is worth going on the train to Key Gardens.