My dad really enjoys family more than anything else. He always says he doesn't want gifts and doesn't really need anything. So we usually just get everyone together (which is kids, grandkids, great-grandkids, quite a crew actually) for a cookout or something casual. He gets lots of hugs and that's what makes him happiest!
His 80th birthday is this month, so I rented a pavilion at a nearby park for the day. There's a playground and hiking trails and he may invite a few friends, but for the most part it will be family. We'll BBQ. He would be gracious but uncomfortable if we did anything fancier or bigger.
My mom's 80th is next year and for her I'm thinking maybe an extended family weekend getaway. Easy to do on a smaller scale too - take your dad away overnight or to his favorite restaurant or for an afternoon outing to a place he likes - hiking, picnicing, museum, civil war battlefield, etc. or just cook him his favorite meal. I don't know your budget but it can be inexpensive.
Unless I've got a really good gift idea I know they'll like, I always think an experience/together time is the way to go. He'll be happy just to know that he is loved and you are thinking of him. That's how most dads are.
