A nice bottle of 12 year old single malt scotch is always a good choice. It should be available for around $50 at a local store.
Dinner, sit down, he'd like to hear from you.
Cook him dinner and spend time with him, that's what my daughter did for my last birthday. It was the best "gift"
You should combine cpgnatly and DozerDan's suggestions.
Sit down, and drink a nice bottle of single malt scotch with him.
spend some time with him and get him talking about random things. it's a gift in itself, and it might spark some ideas... but really, all he wants is your love and respect so anything you get him will be fine if it conveys that.
A card for starters, Take him out to dinner or something too
It all depends on what he likes. Maybe squeeze it into a conversation without being too obvious.
what does your Dad like? sports, cars???