Agreed with Zuikologist.
A few additions, a walk through Carnaby Street. Then head to Antidote afterwards, a lovely French wine bar at 12A Newburgh St.
When in Soho, head to Soho Square, foot of Frith St. Then tapas at Barrafina, 26 Dean St, just around the corner.
In Greenwich go to the observatory, and stand in two different time zones.
You can take the ferry up the Thames from the Borough market on Southbank.
If you like tacos, El Pastor at the market is brilliant!
If you feel like a 20 minute train ride, St Albans is a lovely town. Nice place to walk and lots to see, nice pubs as well.