I agree with the spa treatment. Put a gift card in a "Enjoy your birthday" card. Let her make the appointment. You just cover the cost. Do not give it to her in person. Leave it someplace she will find it.
If I was in your shoes, I would have plans doing something away from her. Give her space on her birthday. You go do something manly.
"What is best for my kids is best for me" Amor Fati Link to quotes: https://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=2879712