Is it just me or are dads really hard to buy for? They seem to have everything and need nothing. Besides that, if your dad is limited on the types of hobbies he does (or if you can't afford the things that go with the hobbies) you're even more out of luck.
But I've got some suggestions for you, that range in type of gift and price range. Don't forget that Father's Day is a good time to honor the friends in your life too. Maybe you have someone that has mentored you for years, or even a friend that is a new dad. These are occasions to do something nice for them as well.