Sharp makes an IR distance sensor that is good out to 5'. They have a narrow detection cone. They have served us well over the years. There is a problem with long leads. A 100 uf cap across the gnd and 5 volts will help. If you try them, don't forget to buy the matching JST pig tail. They do not like clear poly. A different sensing plate would be needed.
http://www.pololu.com/catalog/product/1137