use the Vic line as mentioned above, it'll be a 5-10min journey. Carnaby street is an easy 5min walk from Ox Circus station.
Not sure it'll be how you remember it though.
after that, it depends on what you're looking for.
More shopping ?
Selfridges is a 10min walk (max) up Ox street towards Bond street.
HMV and Virgin megastores on Oxford street, giant Next, and the pricier stores on Bond Street.
Covent Garden is a 15min stroll (there's shortcuts, but) the scenic routes down Regent Street and through Leicester Square. Loads of places to eat round there, at prices to suit everyone.
British Museum (15min walk from Ox circus) is free and well worth a look.
and if you get a bus pass you may as well jump on and off the buses and save some time.
hope that helps.