Go to Camden Town and go to Underworld. Four stops away from Oxford St, Victoria line (change at Euston for Northern Line). Visit museums, theres too much to do in London. I live in Angel, so I am in the city and there's so much to do. Also, Islington/Angel Upper Street and Primrose Hill are nice. Leicester Square and Piccadilly Circus are really happening too. Go to Covent Garden Market, it's got a really nice Parisian feel there and near Bloomsbury too.
Walk along the South Bank from Westminster bridge to London Bridge. You walk past the London Eye, Royal Festival Hall, National Film Theatre, Street markets, The Globe Theatre, The Wobbly Millenium Bridge (with a great view of St Pauls) and Tate Modern. All worth a look. If you are a foodie and do it on a Friday or Saturday end up at Borough Market. Loads of fab food and drink to buy.