The "Over the Hill" theme has always been popular, but you can also opt for a theme centered around the celebrants favorite leisure activity. A sports theme party or a luau is always fun, or take a trip down memory lane with a tribute to the decade in which the birthday person grew up, such as the 50's or 60's.
Arrange a surprise party. Know what your dad likes e. g. music, dance, food, drinks. Invite all his beloved relatives and friends. Create a birthday card with your own hand. Book a bouquet and send him in the morning. Make a slideshow of your father's photos and describe it at the party.
This is a great age to have a party where close family show up. Your father may feel weird turning this age, so make him comfortable with a supporting cast.
Have a "60s" party -- a party themed in the 1960s, a decade your dad will be familiar with. You could go for a Mad Men-type theme for the early part of the decade or Flower Power for later.