MIX flour and baking soda in small bowl. Beat margarine in large bowl with electric mixer on medium speed to soften. Gradually add sugars, beating until light and fluffy. Beat in egg and vanilla. Gradually add flour mixture, beating well after each addition. Stir in oats, raisins and cereal (POST GRAPE-NUTS). Drop by rounded tablespoonfuls onto ungreased cookie sheets. BAKE 8 minutes or until lightly browned. Cool 1 minute; remove from cookie sheets. Cool completely on wire racks. Makes 5 dozen. ----