You may hire an air-conditioned taxi for a full day (8 hours or 80 km, whichever comes first) for Rs. 1,000, and a half day (4 hours) for Rs. 550. Ask your hotel what the going rates are in case they've gone up. You can also call Group Mobile Cool Cab Service (22/2490-5151 or 22/2490-5152; sometimes they take a while to answer the phone—do try again) for a cab with air-conditioning.