Covent Garden often has lots of street performers well worth looking out for, The British Museum, Science Museum and National History Museum are all free (optional contribution). The Portobello Market can take up a good hour (even more if there is filming going on) You can also go and hunt for the "Ghost" stations on the Underground (British Museum - between Holborn and Tottenham Court Road and Down Street - between Hyde Park Corner and Knightsbridge) The instrument shops in Denmark Street and the second-hand bookshops in Charing Cross Road can offer rich pickings(detour into chinatown (Garrard St) for an all-you-can-eat-for-£5 lunch). The Japan Centre in Picadilly and Hamleys in regent street are both pretty good (if you don't buy anything :) )