I gave my husband a "We Love You Because" jar last year that the kids and I filled with slips of paper stating reasons we love him. It was his favorite gift ever! So I like the suggestions of 30 reasons you love him!
My sister always decorates the inside of her husband's car on his birthday with balloons, cards, streamers, etc. You could decorate his car the night before so when we gets off work he sees that. Or have the house decorated the same for when he walks in from work, or wakes up after sleeping.
Good luck! Hope you have a memorable day. My kids thought the jar was really fun, too so hopefully your kids would enjoy telling dad why they love him!