An odd gift but I've always loved astronomy... my kids bought me a star one year. It's not like I can do anything with it but it will carry my name forever. International Star Registry.
Another gift that is basically worthless (as most xmas gifts are).. There are many coin houses that will compile and place in an attractive display a set of US coins minted the year of your father's birth.
If he likes to read, an E reader of some sort can be a nice gift.
A free years membership to Android Forums is a great gift idea and fairly cheap.