Depends what you want, Brick Lane and Club Row seem popular and judging by this thread so is the old Spittlefields Market. On Sundays there is the flower market in Columbia Road, not far at all from Shoreditch.
Personally would head into town and wander round Lincolns Inn or Grays Inn, also off Fleet St is Dr Johnson's house and St Brides Church. Round the back of Kings Cross station is reasonably interesting too, St Pancras Lock on the Regents Canal and wander up the towpath to City Rd stopping off at Exmouth St market and Sadlers Wells
Alternatively walk the other way to Camden Lock and Regents Park, bit quieter than the busy streets