I wasn't going to do anything because of how I feel about my child(ren) not expecting something each time there is a special day for someone else. However, as much as it is a special day for the LO that is about to be born, it'll be a special day for DS as he becomes a big brother.
He will be with one of his Grandma's while FI and I are in the hospital, so we bought some activities for him to do at Grandma's like the Crayola Color Wonder drawing pad, a regular drawing pad, new finger paints and the Crayola Color Wonder markers. He also really loves firetrucks/ambulances/police cars, so I'll be on the look out for a new little one to put in the package. If he were older, I'd get things that related to him being a big brother, but he really doesn't get it. He already has the big brother books and what not.
All of that will go in a personalized tote bag, which will probably say Big Brother Max, or just Max.