City tours (around Rs100) are organized both by the Government of India Tourist Office and the West Bengal Tourist Bureau, leaving from their respective premises around 7.30-8.30am and returning around 5pm. Although you don't get much time anywhere they give you a good feel for the city.
An even better way to explore the sprawl of Calcutta is to hire your own taxi and guide , arranged through the Government of India Tourist Office. For a group of three to four people, it works out reasonably cheap (half-day tours from Rs250 and full-day tours from Rs350).
Walking tours provide a great insight into the historic heart of the city and start from BBD Bagh; contact Footsteps (tel 033/337 5757) for more information.