This year I got my dad an amusing little book of poems written as if by kittens. He really likes cats so I think it's perfect for him. What kind of things do you give your dad? Some years I've had a hard time finding something good that I haven't already given him sometime before.
I only said happy father's day to my mother's boyfriend, which is more than I usually say to him every year. So I guess that it was improvement lol. My three little siblings and my mother bought him a card, and some chocolates, as they do almost every year. Sometime she gets a mug too. My mother also cooked, and we all had a small celebration. It was a good day.
They will probably value spending time with you more than anything else. Maybe prepare a picnic basket and find some nice place where you could have a meal together? Or maybe buy him a fishing license and spend the day fishing? Whatever he likes doing, give him a present that would allow you two to spend a day together doing that.