In reply to whatistechnique?:
The whole South Bank is probably quite good fun? From the view of the Houses of Parliament by Westminster Bridge to the Eye, the famous skatepark under the Queen Elizabeth Hall, the Tate Modern, the Globe, the Golden Hind, Tower Bridge, various food things and performers and stuff as you go...
Maybe one for Wednesday rather than just cooking in the heat tomorrow, though. The big museums are probably the best places to hide from the heat. Pick one of Science, Natural History, V&A, Tate Modern, British Museum, National Gallery etc according to their interests.