If it's a nice day, take a boat trip from Westminster Pier - you'll get to see the London Eye, Houses of Parliament, Big Ben, MI5 offices and more in one fell swoop. The London Eye is also a must see.
Eating/Drinking wise, head towards Leicester Square/Piccadilly Circus. Avoid the more obvious places and stroll a few streets back into Chinatown - there's a multitude of fantastic restaurants there.
For drinks, The Old Surgeon (just off Leicester Square, its on the street behind the big casino on the square) or Waxys (labrythine Irish pub - O'Neills on steroids).
Or head to Covent Garden for the cafes and street entertainers. Avoid the Punch and Judy as it's a tourist hellhole and go to the Maple Leaf (street behind Pizza Hut) or the Porterhouse - both are quality establishments.
If you still have time, Camden Town is worth a look, or take a stroll around Hyde Park itself.