I would go for a long leisurely walk in a beautiful park. Or go somewhere out-of-the-ordinary ... a botanical garden, a science or art museum. Slow-dance together in the living room before going to bed.
Rather than buy a box of donuts, or make a cake, stop and get one or two chocolate-covered strawberries for each of you. Decadent, yet not big enough to set you in a tailspin. If you absolutely must have something like cake, go to a restaurant (not even for dinner) and SPLIT one dessert between the two of you.
Happy birthday! Check back with us and tell us what you did to make the day special.