How can I get my Arduino to recognize a specific object using some kind of sensor?
You can get an RFID reader for $7 from eBay:



Cards are around 30 cents:



I have example code for an entry control system which uses that reader.

The cards tend to activate at around 3 cm to 5 cm, so that sounds about right. Of course they aren't particularly directional, but if you are driving in a certain direction and it detects a card it is probably in front of you (however possibly off to one side). 

You could combine card detection with simple range finding using an ultrasonic detector.