My daughter was 2 1/2 when my son was born last summer. She received a book from her baby brother...I'm A Big Sister by Joanna Cole...with an inscription from the baby..."Kinsey, I'm so lucky to have you for a big sister. Love, Your little brother, Matt" It's a great book that enumerates the joys of welcoming a new baby to the family and the advantages of already being "big." When I read it to her, she always makes sure I read the message from her brother. It was really fun to hear her quote the book in the months after he was born... "He's too little to eat pizza, and apples, and ice cream."
I also bought her a kid sized Moby wrap...I knew I'd be carrying the baby in a Moby wrap, and she loved mimicking me, and carrying her own baby dolls in the same way.
As for keeping the girls involved...there are lots of ways they can help, from bringing you a burp cloth for the baby when you need it to making the baby smile when he's crying. As he starts to move around and get into things, have them be the official toy testers and decorate empty toilet paper rolls for them to test the size of toys for him to play with. Example, Barbie shoes fit through the toilet paper roll and are too small for him to play with. Blocks do not fit through the opening and are safe.
Good luck!