We got our son a "big brother" gift. He was a month shy of three when we had our daughter in October. We bought him a toddler camera. Originally, we planned to give it to him at the hospital, but since I was induced, we decided to give it to him the night before. We told him it was his job to help get extra pictures of his new baby sister. We made a big deal out of the fact that it was a big brother gift. Our theory was that while everyone oohed and aahhhed over the baby, he would be distracted by the camera and not get terribly jealous...and we knew he would get bored at the hospital. I think he took pictures of everything but his sister! (the floor, trash cans, my IVs, etc.), but it worked perfectly, he had something to do while he was at the hospital.
His Aunt filled a cardboard suitcase with things from the dollar store: crayons, figures, cars, coloring books, story books, etc. It was meant to keep him busy if he got stuck in the waiting room...it worked like a charm
My mother sent him some big brother shirts. She bought them at cafepress.com She thought they were all so cute, she refused to get just one.