I bought my dad tee-shirts. He has more than enough fishing stuff to last a lifetime, more cigars than he'll smoke in a lifetime, he doesn't need more booze.
If your dad fishes, renew his fishing linsce (sp), or give him a gc to walmart to renew (that's what I used to do).
A subcribtion to a mag?
If you're at home, do his chores for the day/week. Cut the grass for him, cook dinner, ect.
Take him to a movie or rent a movie and watch it together. My dad loves going to movies but my mom rarely wants to see the movies he does. I took my dad to see LOTR's one year, bought him dippin' dots and coffee.
My dad's the type who buys what he wants, so if it's something he wants/needs, then he has it. I stick with tee-shirts and doing stuff.