Fingers are manipulatives too! I have been teaching my 4-year-old son to add and our fingers have been most helpful ;-). Holding up two fingers on one hand and three on the other then having him count the total, gives him a visual representation of 2 plus 3 equals 5.
Of course I don't want him counting on his fingers when he is twelve, but it is perfectly okay for a preschooler. Besides, he is too young to grasp the concept by looking at numbers written out on a worksheet.
And fingers are free!