Cologne: buy him your favorite fragrance for him to wear on your wedding day - it's one last detail he has to remember.
Necktie: a guy can never have too many neckties. Buy him something he can wear to your rehearsal dinner that way he is prepared and knows what you want him to wear.
Swim trunks: if your wedding weekend plans include a relaxing stay at a hotel with a pool, or even a destination wedding on a tropical island, swim trunks would be a perfect gift to remind your dad or future Father-in-law to make time for a quick dip during all the chaos.
Sunglasses: outdoor vows generally come with lots of welcomed sunshine. Buy him some cool shades to help protect his eyes so he doesn't have to squint through the entire ceremony.
Shave kit: Buy him the right tools to make sure he looks good for your big day.
Pocket square: personalize it with a custom monogram or write a sweet note for him to have on your wedding day.
Wedding shoes: whether your wedding party is all wearing custom chucks or just renting shoes for the tux place, your dad and/or future Father-in-law will appreciate shoes already picked out for him. Maybe even add in extra gel soles so that he can dance all night long. :-)