I pulled out the addition flashcards I had made two years earlier for my older boy. My younger one just started second grade. They are things like 8+9 = 17 or 5+ 7 = 12. He knows them all cold, even though we never did them formally! I'll check subtraction next week, but he probably knows them.
My older boy has a processing disorder learning disability, so we did flashcards (addition, subtraction, etc.) almost daily with him for 2 years. He now knows them cold too but it was a big time commitment.
My little one also knew all 500 second grade words the teacher gave us at back to school night, as words they should know by the end of second grade.