Calvin's Certified Genius Card is a very minor object that only appeared once in a weekday strip.
7/28/1994
The Card's only appearance was shown when Calvin and Hobbes were taking a walk when Calvin said he always carried a card with him. The card reads, "Calvin, Certified Genius" (Although it was really more like a "Certified Genius"...). Then Hobbes realized that Calvin embossed the card using something, which he guessed was a screwdriver.