I love JustUs2's idea of adopting a senior in a nursing home. Many of the residents there don't get many visitors and are lonely. Visiting with a senior on your dad's birthday would be a wonderful way to honor him.
Was your dad passionate about any specific cause like helping Veterans or helping the homeless? If so, you could make a donation in his name to an organization that supports a cause he was passionate about. Or, you could volunteer with that organization.
Be sure to take time to remember your dad on his birthday. It can be painful, I know, but it's important to allow yourself to feel your emotions. If you have a family member to talk to, maybe sit down with him/her and share memories of your dad.