I never celebrated their birthdays but here are some ideas:
1. Get a heart of romaine, open it up a bit so there is some space between the leaves, and fill it with treats like apple, grapes, and whatever your rabbit likes.
2. If your yard is safe for free roam, let him run free for a bit, you might want to put some food around the yard for him to eat while exploring.
3. If you are in a suburban neighborhood, takr him for a walk in his harness around the neighborhood or a park.
I hope this list is helpful and I wish him a happy birthday.