Do you really have to ask blueroom?
shobhit, the easiest sensor to add at the last minute like this is a simple wire bumper attached to a small switch. Put the bumper in the front of the robot and when the switch is actuated you stop, back up a bit, turn 90 degree's and try to move forward again. The easiest kind of switch I can think of is a little roller arm switchs that are used in printers as paper detectors. They have small arms on them and are extremely sensitive.