Here is a free suggestion - What does your dad need help with around the house? What does he like to do with you? Get some construction paper or use your computer to design a few **thoughtful** coupons. Make at least 4 of them [the more the better] and put them in an envelope to give him. For example " 2 hours help with the yard work." that is you offering to help him with the yard, or cleaning up the garage, helping him organize his desk, tools or closet, program the DVR or his ipod...things HE would like. He gets to redeem these coupons with you. Happy Father's Day!
Just get alone with him and tell him that you appreciate him. A father son talk is a very powerful thing. It means more than written words or even a very thoughtful gift or activity. There is nothing like being told your appreciated. He obviously is a pretty good dad if you think enough of him to seek advice in how to express your feelings.
Showing appreciation to someone you love doesn't necessarily mean you have to buy a gift or do something extraordinary. You'd be surprise how doing the simple things in life can go a long way. I suggest washing his car, cleaning the yard, throwing the trash out, or mowing the lawn. A small amount to pay for the many things our Dad does for us.
Every day is father's day (and mother's day too). Continue to respect and appreciate your parents for bringing you into the world. Get good grades and become successful. That's the greatest reward to a parent.