I think buying each father a Fathers day card is a nice small and professional way to celebrate Fathers’ day at the office. Then after work it will be a good time to go out and enjoy with friends and spend good time with family.
Depends on the type of office environment you're in but cake is always a safe choice. Something in a in a guy theme. Local bakery should be able to help. You could also set up office Olympics. Chair races, mystery hunt for quirky items, timed paper shredder contests, pie eating contest for lunch. Lots of things you can do!