My son was four when my daughter was born. We bought him several small gifts to open at the hospital and play with. These included books to read, a book of simple art projects and supplies to make them, coloring/activity books, and some Hot Wheels cars. These things were inexpensive, but he was entertained during the long hours at the hospital. Also, we gave him a t-shirt saying "I'm the big brother".
After my daughter was born, I took my son once a month to do something special - just the two of us. Sometimes we would go out for lunch and other times we spent an hour at the park. Again, we didn't spend much money, but the quality time was priceless for both of us.
Good luck to you and congratulations! It's a pretty big transition for the older sibling.
J.