I would just get something that she really likes, like a special stuffed animal or something... but something for a child, not a keepsake thing...
Which was really a good present for baby to bring with, because then the big sisters could imitate mama taking care of baby with their own babies, rather than fighting over her!
We also wrapped them each a disposable camera to use however they want to. Granted, not all the pics come out but they love to do it and it will make them feel a part of the experience.
In addition may I suggest having your DD bring baby a gift? I found that it was a nice way for my kids to appreciate their baby and show it he/she was loved. They actually had a lot of fun doing it and are excited to show baby what they bought. We just let them pick out some little thing, a teething ring or wrist rattle, that type of toy. Nothing pricey or big.
ETA: I forgot that I also made them some doll slings so they could both carry a dolly like mama will do. Even DS at 5 likes to do this when they play "house". Cute fabric, some rings, easy and small! I know that the Attachment Catalog (I think that's the name) sells them if you don't want to sew one.
I'm also getting them t-shirts . . . I'm going to iron on big brother, little brother, and big sister . . . cool black fonts on white tees for photo shoots and fun!
And though I do photography on the side, I never thought to get the kids a disposable camera so they could document the first days of our family of five from their perspective. Adding that to my list--a fabulous idea!
nicole , mom to 3 boys here on earth 9, 7 and 4.5 and 2 girl's 2.5 and 10/16/11. Always remembering my babies in heaven: Sam (9/7/05) at 12.5 wks , Morgan (2/13/06) at 6 wks , Emeric (8/9/10 at 17 wks) and Pepper (11/26/10) at 8wks.