Rhonda, it's been my experience with my in-laws and other relatives who have hit that milestone that they don't necessarily want anything tangible, unless it's something that could be of meaning to the family. At least that's how it was with my family.
Does he have a large or close family? Perhaps a very nice dinner out with everyone. That's what we did for my FIL's 85th. How about putting together a small memory book? I've seen many lovely journals...perhaps one may be filled with special written remembrances from you and your DH, and anyone else you might like to add. I also like Susan's idea of the gift tree.
I wish your FIL a very happy 80th! How nice of you to mark the date with a thoughtful and meaningful gift.