Little Bits of Love
Spend a day on the golf course with your dad if he likes to golf. Paint or glue little slips of paper with the words "I Love You," "You're the Greatest" or "Best Dad" on a set of golf balls as a gift (or tennis balls if that's his game).
If he's not into sports, you can still surprise him by leaving these "little pieces of love" all over the place—in the car, the dresser drawer, on his toothbrush, in his briefcase, in his shoe and so on. He will have such a laugh discovering them throughout the day.